Most, if not all people will do some type of negotiating. But by listing your offer in the OP's add, then it can become more like an auction. This will also result in the OP's ad to be repeatedly bumped back to the top of the classifieds. So by sending offers via pm, it keeps the OP's AD in sequence with the rest of the ADs....You don't negotiate when you're buying?
I've never heard of this problem before...Unless you are asking way too much, of course.
Are you referring to the furniture thread? You started off at $700, made your way down to $300 and someone offered you $200...I wouldn't say they were way off base by at least asking.
Better yet would be that replies be "turned off" so that people quit bumping the ads buy asking questions, low ball offers, ect.
I had a buddy on here get nasty pm's on here from a guy that posted I will take it. In the meantime he had already received numerous pm's from other guys. The guy accused him of selling it out from underneath him.
A simple time stamp can answer any question of who was first. Some guys will post that an "I'll take it" posted into the ad trumps any PMs. I go by first one (by time) to tell me they want. It's rather simple.