Well, I've never purchased anything cycling-related off of eBay, but I have had one "ugly" experience where I ordered a collector's edition DVD set that was described as "in original packaging" (which should have been a nice, gate-fold box thing), and was sent DVD's that were packaged as individual rentals from Hollywood Video! With the Hollywood labels still on the cases! The nerve!

I was promised a refund, so I shipped the DVD's back like a sucker, and after a few weeks hadn't received my refund after several emails to the seller and opening an eBay dispute and such. I finally filed a claim with PayPal, and within a few days they ruled in my favor, and I got a full refund.

Obviously, my item was worth much less than a bicycle, but it's at least 'proof of concept' that PayPal has a pretty good system in place to protect buyers. However, don't be afraid to ask the seller lots of questions to make sure you know what you're getting into and protect yourself from getting burned in the first place! Any good seller should be happy to answer questions about an item.

I haven't bought a ton of stuff on eBay, but I have been using it for over 4 years now and that's the only time I've ever really had a big problem.

Good luck!