Here in Washington state, the Washington Arms Collectors club sponsors several large shows each year. Generally, 200 tables at each. Quite a few of the vendors are just collectors who rent a table and sell off part of their collection, older members wanting to liquidate, things like that, along with many FFL dealers. You need to be a club member to purchase or sell any firearm at the shows, and your membership comes with a background NICS check each year. The FFL dealers still do the 4473 and another NICS check, but the private vendors are selling only to NICS-checked members.
If the shows would just make a requirement like that, we could tell the anti-gunners to take a hike.