Info from an official eBay website:
What is the eBay Bucks Rewards Program?
The eBay Bucks Rewards Program is designed to reward eBay members for purchasing items on www.ebay.com. If you join, you’ll automatically accrue rewards in the form of eBay Bucks based on your qualifying purchases. Upon the completion of each calendar quarter, the eBay Bucks you earned over that 3 month period will be issued to you in the form of an eBay Bucks Certificate. You can use this Certificate to buy more things on eBay!

What is not considered a qualifying purchase?
Qualifying purchases excludes all Classifieds, Business & Industrial Capital Equipment, Real Estate, and eBay Motors categories (except Parts & Accessories in eBay Motors, which will earn eBay Bucks).

Does this program cost anything?
Enrollment in eBay Bucks Program and participation are FREE. There is no cost.

What changes for prior Beta Program Participants?
Existing eBay Bucks members do not need to re-enroll. The current earn period as well as your account balance simply continues and you keep racking up Rewards.

How do I earn eBay Bucks?
Earning eBay Bucks is easy! After you enroll start shopping on eBay and you’ll automatically earn eBay Bucks on qualifying purchases. Watch your Bucks pile up for the calendar quarter; after the quarter has ended you will receive an eBay Bucks Certificate for the total amount earned.

You can learn more at: http://pages.ebay.com/rewards/faq.html