eBay is now testing the "eBay Bucks Reward Program" with select eBay shoppers. For this trial period, enrolled shoppers earn eBay Bucks on qualifying purchases, from May 15 through August 14. Then, they will then have 30 days to spend the eBay Bucks on eBay.

Purchases in certain categories are excluded from the promotion, for instance, all categories in Motors except Parts & Accessories; Classifieds; Live Auctions; Giving Works; Real Estate; and Business & Industrial Capital Equipment.

You can read the Terms here:
http://rewards.ebay.com/terms.jsp

If you go to http://rewards.ebay.com you will be shown a message that all trial participants have been chosen, and you can click a link to be included in further trial testing.