Have you ever been interested in buying an item on eBay that is available as "Best Offer", where there is multiple quantities on offer and some already sold, and you want to see what the other people have paid?
eBay want to hide this information, of course, but it is revealed in certain places.
The first place to look is in that seller's completed items. On the item page, click "See other items" which will bring up a list of any other items that the seller has listed. On the left hand side, under "Show Only", click "Completed listings". You may need to log in at this point. This will bring up a list of items from this seller has finished. Once an item has finished all of the best offer prices will be visible in the history (click the "x sold" link next to history in the "Other item info" section of the completed item description page) so you will be able to get an idea of the seller's limits.
However there is an even better way to see what offers have been accepted for an item that hasn't finished yet. Simply look at the seller's feedback. Assuming the seller hasn't set this information to private, the best offer price is shown immediately when a buyer leaves feedback.
That is a great advantage to buyers but not to sellers, and a significant flaw in the eBay system in my opinion.Update:
eBay have now put a stop to this unfortunately, and no longer display the price. However they do still show that a best offer was accepted, so you can get an indication whether the seller is open to offers.