You have to have the right shopping cart, go to
CartManager - How The World Shops On-Line they have the option to link your google API to the shopping cart, also the shopping cart allows you to link to ebay in the "Extra features section" Now when I use the word link not in the essence of sharing information but giving authorization to use the shopping cart with ebay just like bidpay. I called the shopping cart provider to make sure of this, After you completed all of this, when someone wins a auction from your ebay account...the shopping cart will automatically send them a invoice from google checkout to there email. I also found out that the same shopping cart is compatible with Paypal...this might be a way to link your ebay to paypal without linking them together. I'll check more into the paypal situation.