Hmmm you guys seem to think a lot about Google, not saying that I don't
If they had their own site like eBay, it's true that they could maybe eventually over take eBay and get it popular and known very quickly, but why would they do that if eBay is paying them (must be loads) to put their ads up everywhere? (If I am correct on that note of course).
Also Google could just beat eBay by not charging sellers so much, by not taking a cut out of the final sale etc. etc. And Google is known to be very original, so I am sure they would have great original ideas for a MarketPlace or alternative to Paypal if they ever make them...