A used bicycle can offer good value, but its price alone says little about the frame. Look first for cracks or serious dents, especially near welds and the head tube. Components can often be replaced; a damaged frame may make the whole purchase uneconomical. Ask whether the bicycle has been crashed or modified, too.
Next, feel for play around the steering, wheel hubs and bottom bracket. Lift and spin the wheels, listen for unusual noise, and gently rock the bike with the front brake held. If you are unsure how to judge wear, bring an experienced friend or arrange an inspection at a bike shop. Add likely repair costs to the asking price.
Before paying, compare the bicycle with the advert photos and ask the seller about its origin, service history and proof of ownership. Record the frame number and keep a written receipt. Evasive answers or an inability to inspect the bike in person are good reasons to continue looking.