What do you do if you find your stolen bike's frame on ebay?
To determine if the bike is indeed yours, you could contact the seller and ask for more information about the bike and for more pictures in the hope that you'll get one with a visible identifying mark.

You'll need to be careful. If the seller know's it's stolen asking for more information and pictures could tip them off that you are the rightful owner with the intention of retrieving your property.

You should read the Ebay Stolen Property Policy for what to do if you are convinced the bike is in fact yours.