Ryan & Tron are the only ones able to implement such a thing, & i am also unsure if our current web host/board supports a validation feature like that.
In addition, imagine if a user needed a quick sale (It has happened), and no mod is available for a couple hours.
Its a bit more work for us mods, and it swings the ball our way meaning if we dont like certain users, we can deny their for sale threads. It would also remove a bit of you guys freedom in the sense you can post anything you want to sell here (providing its within reason(think: illegal items) & that you meet the rules).
Thats just my $0.02 on the matter though.