If you're a licensed dealer, it's a
federal license to sell firearms and you are required to do a background checks on all sales regardless of the state you are in. If you are selling a gun or guns to a non-FFL holder to make a profit and you do not have an FFL yourself, you are breaking the law.
So basically the only sales that don't currently require a background check are transfers or private sales for no profit. And even that is difficult and laws exist like, you can't ship a firearm across state lines unless its going to an FFL holder.
Every booth at a gun show requires a background check. The "loophole" is people selling their guns in the parking lot which is probably already illegal but honestly doesn't happen. Why would vendors pay to set up a booth at the gun show if everyone just buys black-market guns in the parking lot.
edit: just saw this on facebook after I posted the above...
https://www.youtube.com/watch?feature=player_embedded&v=dpcjp5QagII