You should read this piece on offer and acceptance law to explain why in this case the seller is not obligated and there is no contract yet.

http://www.west.net/~smith/acceptance.htm

http://www.duhaime.org/contract/ca-con4.aspx

our system is such that an offer must be accepted by a seller before it is binding in any way.

also to answer your note above let me give anothe example. say you want $100 for your motor. you list it in classifieds. not auctions because you don't want an auction. You want $100 or best offer. You wake up tomorrow and find 5 offers. two of them are at $90, one is at $100, one at $110 and another at $120. Which one do you take? Will you refuse all because guys offered more and you did not know they would and now you realized you should have listed this as an auction? I think you see the point. A seller cannot control high offers and cannot know when to list as an auction. Heck if a seller thought he could get MORE and get a bidding war going I think everyone would go that route. In fact more people should as our auctions for certain products can do extremely well based on our sell through data here.