i've had good and bad experiences on ebay. my fixie was an exceptionally good one- bike shipped from new york, cost me $25- for shipping, and the seller did not claim his paypal payment until the bike arrived. he provided me with a ups tracking no., and sent follow-up emails. i've bought several other fairly expensive things (like my fat ibanez 6-string bass) with equally good luck. the crappy transactions are usually from small, low-dollar items from high-volume sellers- impersonal and automated all the way. i've also had some stuff just not show up. ebay and paypal's buyer protection is a frigging nightmare. paypal credit acct. is a little better for following up on claims.
just don't buy a high-end bike for too good a price from a seller who insists on western union.