: Inspect for any cracks, dents, or deep rust, especially on steel frames. Carbon fiber frames require extra caution, as they can harbor non-visible structural damage that is expensive to repair.
: Examine "consumable" parts. While brake pads are cheap to replace, heavily worn rims or a stretched chain can lead to costly repairs. Use a chain checker tool to assess if the drivetrain needs immediate attention.
When evaluating a used bicycle, the frame is the most critical component. Unlike parts that can be easily swapped, a damaged frame often marks the end of a bike's life.