I thought the same thing but probably not possible. Firstly, the promoter must have insurance and no insurer would stand for such shenanigans. Secondly, if sales of weapons occur, even at a "Barbie Doll Convention", the BATF and LEA agencies would quickly find out and come down hard.
See, when pols and police chiefs hold "no-questions-asked 'Gun Buybacks'" and don't do the necessary paperwork, that's OK...even though they are committing multiple federal felonies (or so it seems to a non-lawyer)...
Oh well.
--Boris