Here are a few tips to avoid buying a stolen bike.
Check out the seller. Look through their ratings carefully. Are they new to eBay? Have they sold a lot of bikes in a few days, using different excuses for each sale? Do they fail to respond to buyers' questions? Are they postage-only, or 'meet me down the pub'? Do they go for private listings?
BEWARE. If it seems like it's too good to be true, it probably is.
REMEMBER: buying a stolen bike encourages thieves. Next time it might be your bike that's stolen.