Basically, link (completed in the bing application) a bing account (free) with a paypal account (also free), search for an e-bay item on bing (find it first using regular ebay, then search again though bing), a small dollar sign will appear next to the auction if it is bing eligible (meaning it was searched with bing AND it is buyitnow AND they accept paypal), pay for your item with a paypal account linked to your bing account then wait a few minutes and you will get an e-mail saying how much cash you will get transfer to your paypal account after 60 days.
The normal cash back is 8%, but I have seen it higher, just depends on the day. Also, you can use other merchants besides e-bay, but I never have. Just read the links I posted, or check it out. THey really do pay!