Vast majority of sales at gun shows are through FFL dealers. Everyone that has a booth at any gun show I know of is REQUIRED to have an FFL if they are selling guns. Of course not all booths sell guns there at all.
The thing is, many people like to bring a gun or two with them to the show with the intent to sell it off as a private sale. They sling a nice modded AR15 around their shoulder and walk around enjoying the show. Other people at the show see them and invariably ask if said firearm is for sale. Most gun shows don't let them do private sales inside the area the show is being held. So they get a stamp on their hands, walk to their cars in the parking lot and do a private transaction sale.
Other people prior to a gun show and looking to sell off their privately own guns will list those guns for sale on some site like craigslist with the intention to meet the other person at a gunshow parking lot to complete the deal. For whatever reason, people are more comfortable selling a used privately own firearm that way. Been doing that for a long time, just before the ads were in the newspaper personals section instead of craigslist.
That is why the coined termed "gunshow loophole" is just ridiculously bad. There is no loophole at all. Private sales without background checks do not happen in the gunshow area. Private sales, no matter where they occur, have never been required by the Federal, State, or Local governments to use the NICS background application to complete a private sale where such a sale occurs at. Right now though to use the NICS system, a person is required to own an FFL which is not cheap to get. Hence why people charge for that service. I have no problem opening up the NICS system to allow free background checks for gun sales between private individuals for free at all though.
To also clarify something else. If you are licensed dealer. IE you own an FFL, then you are never allowed by law to do a private sale of a firearm. Every firearm you sell must have a background check applied. Failure to do so is a strict penalty.