I wouldn't mind a fee for posting in the classifieds. More than a one time fee or minimal annual fee might be a turn off for a lot of honest but casual, infrequent, or impulse buyers/sellers though. Just because someone isn't part of the long-time posting fraternity doesn't mean they aren't straight up people.
I am not interested in posting more detailed information on my public profile than I am now. I communicate all particulars to the individuals I deal with in private, but don't want to advertise my full name, address, phone number, ect. on a public forum. Might as well leave the door open, lock up the dogs, and pack up all the good stuff for the burgler...