We already ask for proofs before approving any selling topicsthe middle man approach is smart , but you can also make a rule that anyone willing to trade/sell should show proofs of having the invites he offers to the moderator or the owner , failing to do so may cause ban
i saw this in another website , and they ban scammers before the scam is done
But sometime scammers pm victims before posting any sell topic and we can not watch pms