I admin on another site and our classified rules state people can ask questions about the item, offer a price or offer to buy (which normally then goes to private message for shipping details, etc.) This way, basic questions can be publicly addressed and save the seller from getting the same question 5 times via PM.
But at no time can people post about anything other than the item for sale. If they do, they may get a warning or they may get a short time-out. People just don't pay attention to stated rules regardless of how well or obvious they are posted. It sinks in when there's a penalty.
The whole idea is like what's been said: Inadvertent bumps by buddies, it's not a "discussion thread", or worse yet, someone posting the item is "cheaper" at XYZ site. People that want to just "comment" are instructed to PM the seller or poster asking the question.
It does take an active Mod. You can't justify a penalty unless it's evenly/consistently enforced.