Buyer-driven ebay

As far as I can see (and I haven't researched much), there is no buyer-driven version of ebay. What I mean by this is a site similar to ebay where buyers log on and post items they want and how much they are willing to pay. When a seller comes to want to sell an item they input the details into this site and find people who might be interested. The seller then makes an offer, which is contractually binding when the seller agrees.

The problem this solves is the temporal disconnect between buyers and sellers, which is more common for rarer, less traded items. For example, a seller may put something on ebay and have no response as the person that is interested in that item hasn't logged on recently to check.

Possibly the best mode for this would be an add-on to existing auction sites so that sellers are automatically informed of interested buyers when they start to submit an item to the site (but before they set it up as an auction).