I'm not a huge fan of eBay, but they've added a functionality to their "make an offer" sales that seems like the best solution. It's just an email filter that immediatly bounces back a decline offer if it's below a predetermined amount. If the offer is above the floor, the email goes on to the seller and he/she has the standard 48 hours to accept/decline. It seems to be the best way to weed out offers you absolutely won't accept.
