Why the hell doesn't Google show you the purchasers email address?!?! Is that email address that they do show one of those email addresses that redirect to the real email addy, like with Craigslist?
If you use Google Checkout you're basically giving your customer to Google. Read all the stuff you have to agree to before putting it on your site and you'll see what I mean. The e-mail issue is just the tip of the iceberg....