ATF has guidelines for what constitutes "Engaging in the business" of selling firearms, though has no concrete definitions on how many guns one may sell.  The new "advisory" they just published as part of President 0's new "gun control" push simply reiterates exactly what they already say.
Official link is here: https://www.atf.gov/file/100871/download
Here is a layman's summary of the matter:
To buy new guns at wholesale, one must have an FFL--Federal Firearm's License, which come in several flavors for selling, smithing, manufacturing, ammunition, explosives, and with addenda for import or restricted National Firearms Act weapons--silencers, machine guns, short barrels, destructive devices and certain oddities.
If one plans to "Engage in the business" of selling firearms, new or used, an FFL is required.
A PRIVATE CITIZEN not "engaged in business" may buy and sell guns for purposes of collecting or using.  If you get tired of your old XDm and want a Glock, you can sell your XDm to anyone who is not a prohibited person (felon, domestic abuser, drug or alcohol abuser, and not under 18, etc).  If you decide you want to sell off your old revolvers and upgrade to newer pistols, you can do so.  If you decide you're 80, have done all the shooting you're going to, you can unload your collection without a license.
If you sell online, you can either meet a resident of your own state face to face and see ID, or, you can send it to an FFL who will log it in, transfer it to them, and conduct the background check, for a small fee. If they are not a resident of your state, you MUST follow this method. If it is a longarm, they may, as long as their state allows, receive it from an FFL in your state.  If it is a handgun, it MUST be shipped to an FFL in their state.  You can't even hand it to said FFL in your state at a gun show for him to take back to his state.  He must receive it via common carrier.  Don't ask why this is the law. It just is.  THERE IS NO "LOOPHOLE" FOR SELLING GUNS ON THE INTERNET. FEDERAL LAW APPLIES. Enforcing it is another issue, but the law is the law.
Every major online firearm site is even set up to tag "FFL Required?" with YES for firearms and NO for accessories.  They patrol their listings regularly, and very few people will risk selling a gun without it going to an FFL in case of a sting. Trust me on this, there are sellers who won't even sell to collectors, or demand an FFL for non-firearm items out of paranoid fear.  I'm sure there's a seller on Arms America somewhere who'll agree that for an unmarked MO or Western Union, he'll ship to your apartment.  Good luck finding him. And if you do: He's willing to violate the law to do this. You're willing to violate the law to do this. He probably has a PMB box somewhere under a false name, and may just keep your money and not ship the gun. It's not as if you can complain to the cops that he wouldn't ship your gun illegally.  FFLs actually offer protection to the buyer as well as the seller, to ensure both parties are legit.  If it wasn't necessary, few sellers would bother, though depending on the value of the gun, they might. But, since it's the law, there are benefits with the hindrances, and you make use of them.
Yes, some such deals go through, and always did, and always will, and no law is going to stop them. There are too many USPS, UPS, FedEx and DHL packages to ever search even 1% of them, and there's no guarantee the shipper's name and address is real.  Online funds and barter work as well as Western Union and gas station money orders always did. Just because we've had these laws for almost 50 years doesn't mean people comply with them, just as they don't comply with bans on selling pot or coke.
 
Exempt from the "only through a dealer" or "only via common carrier for handgun" are certain collectibles known as Curios and Relics, which ATF keeps a list of, or, are 50 years old an IN THEIR ORIGINAL CONFIGURATION (not with a different stock, shorter barrel, etc).  Any mods reset the 50 year clock for purpose of being a C&R only. There is an FFL, the Type 03, for C&R collectors. If you don't have a C&R FFL and receive one, you can do as you wish within the law. If you have a C&R FFL, you can receive such items directly by mail or carrier to your home of record, and must keep a log.  If you have logged the weapon as a C&R you MAY NOT modify it with aftermarket stocks, etc, that change its format.
A C&R weapon may go to any FFL anywhere at any time. So, the seller WOULD be able to transfer it to an out of state buyer who was in state, who had a C&R FFL, without having it shipped to a receiving FFL.
Clear? I hope so.
Now, a C&R holder is a collector, and may, for example, buy a crate of 20 Mosin Nagants, Russian surplus, look through the crate, keep 5 with arsenal marks they need for their collection, and sell the other 15. That the 15 sell individually for more than the buyer paid is not of itself "engaging in business."  Their purpose is to build a collection, and their documentation will show it. Most such collectors will then plow the sale money back into more guns for the collection.
An important but not widely known exception to all of this are weapons made before 1899, or using black powder, or not using fixed cartridges (such as pinfire).  When the Gun Control Act of 1968 was passed, those were exempted due to their age and function. A firearm is legally defined as firing a fixed cartridge, and manufactured on or after Jan 1, 1899.  There is a regular trade in antiques among both collectors, and shooters who prefer anonymity. Pre-99 and non-fixed guns may be modified within the limits of the National Firearms Act (you may not cut them short, silence them, or convert them to full auto), because they are not firearms under the GCA. They can be shipped without transfer papers.  It is important to note this is a federal law only, and most states still make felons ineligible to own them, and some states require dealer transfer on all items that shoot. THIS LAW APPLIES OUTSIDE AND INSIDE A GUN SHOW. There is no "loophole."
It is probable that the value of paperwork exempt antiques just increased with the greater scrutiny that will be placed on private sellers, since ATF officially may not, and does not want to, take notice of them.
 
So let us consider several sellers at a gun show.
First is a dealer with an FFL from that state. He can buy and sell, and transfer to anyone in state, and to long gun buyers from out of state (as long as their state permits). THIS IS EXACTLY THE SAME AS IS REQUIRED OUTSIDE THE SHOW AT HIS PLACE OF BUSINESS.  There is no "gun show loophole."
Next is a dealer from out of state, who may exhibit (usually high end collectibles) and arrange to sell and transfer via common carrier, through a dealer in that state. Again, he can't just hand the guns over to that dealer. They must be shipped.  THIS IS EXACTLY THE SAME AS IS REQUIRED OUTSIDE THE SHOW AT HIS PLACE OF BUSINESS.  There is no "gun show loophole."
A C&R holder may buy, and sell to anyone in state with ID, or anyone from out of state with any kind of FFL--dealer or collector.  However, this collector must be able to document his intent to collect.  Let's say he has 15 Mosins on the table, as mentioned above, and has a sign, "Looking for Finnish Mosins and parts," or "Want to Buy SMLEs, Mausers and related militaria."  Or let's say he doesn't have the sign, but an educated observer can look at him and say, "This guy's a C&R looking for deals and selling off the old stuff."  THIS IS EXACTLY THE SAME AS IS REQUIRED OUTSIDE THE SHOW.  There is no "gun show loophole."
A collector, without a C&R, can do what the C&R holder above can, but may not buy anything from out of state without having an FFL DEALER transfer it to him.  A C&R holder may buy and sell, but MAY NOT transfer, because he can't access the National Instant criminal background Check System.  The collector may have a couple of old Colt Special Police revolvers, an old NYPD Glock, a couple of 1960s shotguns and a Mauser.  He also is buying and selling, but he's obviously looking for specific things.  Someone walks by with a 1970s Colt Gold Cup, he might buy it. If they have a .25 Jennings for $30, he ignores it, because it's junk and he's not interested. THIS IS EXACTLY THE SAME AS OUTSIDE THE SHOW.  There is no "gun show loophole."
A private owner walking around, who's looking to unload some old guns for new guns. He has a half dozen older revolvers, and a sign, "Want to Buy Glock 23 or XD .40 cal."  Again, he can only sell to in-state residents, or ship to their FFL out of state.  THIS IS EXACTLY THE SAME AS IS REQUIRED OUTSIDE THE SHOW.  There is no "gun show loophole."
A retired guy who's got a table with a metric crap ton of Mausers, Mosins, 2nd Model Smith & Wesson revolvers, Winchester shotguns...and is selling them because he's retired and needs money.  He's not buying replacements. He's just unloading. THIS IS EXACTLY THE SAME AS WOULD HAPPEN OUTSIDE THE SHOW.  There is no "gun show loophole."
A dealer in antiques and black powder firearms, who has only historical firearms made  before 1899, or black powder originals or reproductions. He does not need an FFL and never did, because under the law, what he is selling are not firearms. THIS IS NOT A LOOPHOLE, THIS IS FEDERAL LAW.
Now we come to the joker.  He has a table with a dozen mixed guns on it.  He sells some. He buys others. He sees the $30 Jennings and buys it, sticks a $40 tag on it, and puts it back on the  table. THIS guy is "engaging in business" without a license. He's not selling many, and likely not actually making a profit after table fees and gas, but his intent is to sell guns for more than he paid for them. That's the "unlicensed dealer." 
Or is he?
He might also have a collection, and be using the funds from that sale to build his collection. So what's the call?
And the call is, "I would know if I looked at his table, and so would an ATF agent."  Much like porn is, "I know it when I see it." If he has a business card, it shows intent. A sign reading, "Always buying and selling guns" MIGHT show intent.  THIS IS EXACTLY THE SAME AS IS THE LAW OUTSIDE THE SHOW.  There is no "gun show loophole."
There isn't a clear way to define a volume for "engaging in business," especially since firearm law is part of the Tax Code, and any change to it will be exploited into other items of trade for purposes of collecting or avoiding tax. 
As part of the Prez's latest push, ATF has reiterated their existing rules, and it's possible they'll follow up with more agents at gun shows (And there are pretty much ALWAYS ATF agents at gun shows.  Most people in ATF like guns and buy for themselves as well) (Our local show has agents on hand to answer dealer inquiries).  Those agents will have to make those calls based on available evidence.
And the unlicensed dealers?
The agents will know them when they see them. Just as we all do.
But as before, exceedingly few criminals walk into a building full of cops, feds, veterans, licensed dealers and hired security looking to buy a Hi Point Fotay to do a drive by with.  There are much easier ways to get a gun.
So, literally nothing has changed.
The emperor has no clothes.
EDIT:  oh, to note--you can sell as many guns as you want, transferred through a dealer, at any profit margin you can get, and it's not "business," because you've involved the dealer...even if he makes $10 on every transfer, and you make a half million selling off a huge estate.
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Edited on 6 Jan with clarifications on antiques and minor style changes.
