There are a few important questions to ask and things to look out for on second hand bikes. First of all find out what year model the bike is so that the correct age can be determined. This will also give an opportunity to check the original spec and cost of the bike when new. Question the seller about how it was used, how often ridden and how many races and events. Compare this to the bikes age and the activity of the seller and see if it all fits together. Ask about how the bike was looked after and serviced and any upgrades. The biggest risk in buying a used bike is that the frame warranty is typically not transferred from the original owner. So it is important to check the frame for any scratches or dents or cracks that will be a problem. If it is an expensive bike then get a reference from the bike shop where it was serviced last. If not consider having your local bike shop check it out if possible.