in For Sale, Want To Buy, and Group Buys forums, I've done two different methods.
Have the users prefix their posts with -
The other thing was to create a category called "Private Party Sales". Then create three sub forums called -
Want To Buy
The first option is nice if you have very few new topics
The second option is nice if you are busy and want to make the topics easy for everyone to deal with, without having to ask that the OP (or a moderator to edit) write a prefix in their topic.