Buying bike components on ebay is pretty straight forward. Trust your eyes. If the guy says the derailuer only has 200 miles on it and it looks like it's been through world war 3 pass it up. As for bike frames, get the persons phone number and ask direct questions. If the bike is accurately discribed the seller will be open and helpful. If it's a high buck frame ask if you can pay using ebay's escrow service and that way if the frame is damaged or not as descibed you can back out but you must pay shipping charges.