Roughly you can pay for items using your google checkout account where the retailers show the google checkout logo. So you don't have to enter your payment details multiple times on various sites etc.Makes things more secure, keeps your info safer. Their % is much better too.
Ebuyer are also one of the included merchants. So if there's anything £30 or over there your after and pay using google checkout you'll get £10 knocked off. As far as i'm aware this is not a use once offer - so roughly you can fill your boots with multiple orders
There's a special way to go about ordering using google checkout though, to make sure you get the £10 discount.
"Offer Conditions: To earn a £10 discount, buyers who haven't signed up for Google Checkout in the past must sign-up for Checkout on Google or on a Google Checkout merchant web site during the promotion. To redeem the £10 discount, buyers must make a purchase of at least £30 Inc Vat, (not including shipping), and other discounts from any Google Checkout merchant. Google reserves the right to cancel discounts or take other action."