Thats the promoters deal. I would think sellers are welcome to do whatever they want with the info, ideally burn it but I'm sure some paranoid individuals keep them forever in the unlikely event the police ever come ask what happened to a gun they papered.
Dealers need to keep the 4473 and handgun forms obviously, but the rest of us are exempt from record keeping, legally.
Some shows require colored stickers on the weapons you bring in and a seperate color for one you sold going out. Its an effort to keep stolen guns from leaving the show, but Im not sure how they can tell the difference between one someone brought in and didn't sell and ones that been stolen, as they would both have the same sticker on them. Not to mention the cheap stickers they buy seem to always fall off the guns.