Personally, I'm on the fence about idea #2. Yes, it would eliminate thread crapping. However, I don't know how others feel about this, but I don't see anything wrong with positive comments in a for sale thread. In some cases it probably helps generate a buzz about the item, possibly leading to a better outcome for the seller (i.e. full asking price, bidding war, etc.). Having said that, I realize that threads being bumped because of useless (yet positive) comments such as, "if my mom bumped my allowance I'd be all over this," annoys the shit out of some people. It's a fine line I guess.
Other things to consider are sellers' inboxes being full (nothing more frustrating when you're trying to buy something and can't reach the person), and the loss of the ability to call out known scams/scammers.