There may be a few that feel this is unethical, and it IS against eBay rules. But some of the most respected and high end collectors have added some of there best marbles, boxes, etc. by using this method. It is rampant, and you can't stop it. So your choice either join in, or win auctions once in awhile where the seller also has ethics and lets the auction run it's course. Frankly, if a seller is dumb enough to not see through an under the table offer, they deserve the lower price they get.