I don't know if this has been discussed here before but I just found out about it.
Go to www.live.com
and sign up for an account. Do a search for the products you're looking for on Ebay (or anywhere else for that matter) when the google style list comes up pick "live search cashback on Ebay" which then loads the Ebay listings which meet your search criteria. For example get 20% cashback on new Canon lenses when you buy them using a buy it now and pay via paypal.
Microsoft puts the cashback in your paypal account in 4-6 weeks. I've got $276 (30%) coming back on a Trijicon ACOG I've been wanting for my AR.
Just make sure you have a live.com account, a paypal account and an Ebay account and you must go through live.com to end up at the ebay listing you're interested in.
It looks like they're giving 20% cashback on all Canon/nikon stuff. You can use this feature up to 12 times.