If I might offer some suggestions:
Minimum post count, not just member time. Don’t publicize the post count number either, and they must be “contributing posts” not just “+1,” “I agree with that,” etc. Very few folks will put in the time and energy to do that to try and rip folks off.
The sale forum is locked and not viewable unless you have access. This also helps keep folks from trying to rip anyone of as they have no idea if it’s worth it and thus they go elsewhere with easier pickings.
No questions/comments/complaints in the thread about the terms of the sale. You can ask all you want about the item however. If you want to try and haggle a better deal, or you feel the shipping cost is too high etc do that over PM.
Explicitly state in the thread TITLE where you the SELLER are, so people can see quickly if it will be an international sale.
The terms of purchase, method of payment, etc are all handled through PM and are between the seller and the buyer exclusively. has no hand in any of it. I’d make a sticky listing the buyer/seller protections of various internet payment services.
Have a seperate feedback thread in the forsale section that buyers and sellers may post in regarding their transaction. This is not a b***h’ing forum, rather it’s to say if it went well or poorly. This needs to be mod’d fairly closely to avoid to turning in a mess on occasion (from past experiences).
Have a terms of usage for the for sale section that removes from anything and everything going on in the the F/S/T forum, except keeping the threads properly formatted, and the feedback forum mod’d.
Make sure everyone’s very aware that the forum can go away in short order if it turns into too much trouble for the staff.