I like the way the classifieds are handled on another forum I participate on occasionally. Only the seller can post in his thread. Therefore all communications must be via PM, email, or phone, but not in the thread. So no "buddy-bumping" occurs. Bumps allowed at 48 hours. The poster is locked out from editing a post after about 90 minutes. This means the asking price cannot be deleted. The seller must post a price or the thread is deleted.
The plusses to this are that with only the seller posting, it makes it very clear to the mods if the thread is being bumped too frequently. It benefits all members because they can research the recent asking prices of any item they may be interested in. One can say that the price is private between the buyer and seller but if you are using a public forum to make a sale then perhaps all members have a claim to want to be able to access that information. Besides, the actual final price does remain confidential. Also, requiring the seller to post a price eliminates the blind auctions that result from no price "make an offer" listings.