http://www.thefiringline.com/forums/...ad.php?t=71290
My understanding is that The Firing Line "Gun Show" is provided as a service for the members, not as the focus of the site. The standard rules for posting apply, but there are no special rules for how the transactions are carried out.
There is a link in the thread I linked to above that should help you determine the legalities of firearms purchases and trades. Other than that and the guidelines in the thread I linked to above, it's pretty much between you and the person you do business with.