Check out Dawdle (http://dawdle.com). A friend of mine started it, its a pretty nifty E-Bay like site based around video games which takes a stab at solving the credentials issue.
The only site selling stuff that I know of, and actually tackles / comes closest to solving the credentials issue is Facebook. The only problem with them is that I don't see that many listings compared to craigslist and ebay
As a seller, you don't care who your buyer is, only that you can get paid and that the buyer agrees to have it shipped reasonably. We take shipping choice off the table - everyone has to ship USPS within the US only (no foreign transactions yet, we'll add this choice later, but that's the brute force way to solve it for now).
I wouldn't have even known that you guys have system/mechanism of guaranteed payment (and even delivery?), had you not posted it here