ideally, maybe you should be allowed just the one posting in a sales topic, after initial posting it is locked automatically, only thing you can do is edit your initial post? that would stop the bumping and everyone would be equal. Personally don't think bumping improves the sales, those interested in buying will look at all the posts.
Maybe an automatic delete of the topic after a month or two???
I would also ban sellers form listing three records in three separate topics, put them all in one. Doesn't matter how big the record it will still get viewed.
PS. oops just seen Simsy's post! I would only allow a genuine question, condition etc, The "pm'ed you" and "pm'ed you back" posts are the most pointless of the lot