Microsoft is promoting use of their "Live.com" site and offering discounts of 10% to 35% off all qualifying Ebay items that are listed with a "Buy It Now" option that are paid for using PayPal. The current discount is 20%, but it changes without notice. The search needs to originate from Live.com, and result in a qualifying Buy It Now purchase through Ebay, and needs to be paid for using PayPal. You will see the Microsoft Cashback symbol if the item qualifies....most "BIN" items qualify...the trick is to use the Live.com search to get an Ebay hit that has the MS Cashback symbol...from there, you can search Ebay for all kinds of qualifying items. Basically you need an ebay account, a paypal account, an MS live.com account (you can use an existing email account to open the live.com account). The eBay item must be a "Buy It Now" item. You can save up to $250 per transaction, and can use the discount 3 times per Ebay account.
I've saved $118 on a desktop PC, $140 for my son's college laptop, and $250 on a Shop Fox W1677 w/free shipping...it's pretty slick. The details are pretty straight forward, the restrictions are generous, and the discounts are attractive IMHO.
There's some explanation here, or on Fatwallet.com, Live.com, or on Ebay.
Look for this pic: