I had a bit of a look around when I first saw it. It seems to be on very popular items, such as current books, computer games etc. It does not appear on items that are a little bit more uncommon. There is a little tiny question mark icon next to the price indicator which gives a run down on how ebay arrive at the figure.

This would work for new items, but used items and collectables can vary in condition and price to a huge extent, so it could be very misleading for these items.

Ebay seems to be taking a similar stance to the big supermarkets. ie. drive the price down as much as possible for the people supplying the product ( in ebays case the ebay seller ) , keep the suppliers under the thumb and provide cheaper products to the end user / consumer. In ebays case this is the buyers.

This would be important for ebay in its quest to compete with companies such as Amazon.