I understand you need to be a donating member to sell from the classifieds, and this my have already been covered by what about WTB threads?
Do you need to donate to post a WTB?
i believe so. the donating members are a seperatre group in the forum db. and you can set the read/write permissions for that group for all forums and sub forums. there isnt a way to classify a user to make a specific type of thread. You can either make one or you cannot