I banned 3 thieves this week on a forum. People trying to scam a forum member that had something for sale, or wanted to buy, in classifieds. The criminals in every case tried to do their dirty deed by taking everything to PM immediately and not post publicly where the other forum members could see the interaction. Happens ALL THE TIME that way. In two of those cases the azzhole thief sent a PM to the poster and said "contact Bill, he's looking for that. His email is blah, blah, blah".
Thieves like to operate in darkness...more eyes on posts in classifieds have several benefits this being one of them. Sorry man...this "professional versus unprofessional" schtick is just plain odd. Not thinking interested parties should be able to ask a question of the seller is even odder. Makes me suspicious of you based on experience.
There is also an audit trail when posts are public. We never delete classifieds transactions, rather move them to a dump forum. They can always be recovered in the event of dispute. Can't do that with PM's.