eBay has barred its users from employing Google Checkout on its site.
eBay’s new Acceptable Payments Policy states “sellers may not request payment through online payment methods not specifically permitted in this policy,” and the document does not specify that Google Checkout is an acceptable payment method.
The reason behind this? as it looks obvious to me, the reason is Ebay's own PayPal. Google Checkout was rumored as a "PayPal Killer". And, it is the one greatest ever competitor to PayPal.

There are over held a dozen requirements that Ebay requires of payment methods in order to get Ebay approval, including a track record of providing safe and reliable financial banking.

So, no Google Checkout on Ebay now, or maybe ever....