Quick answer
Do not buy or move a used piano until you understand whether it can hold tune, whether the action works, whether humidity has damaged it and whether the moving cost makes sense.
Major red flags
- Mildew smell, water stains, swollen wood or rust.
- Many sticky, dead or uneven keys.
- Buzzing, rattling, missing strings or broken hammers.
- A seller who says it has not been tuned for many years.
- Piano stored in a garage, shed, damp basement or unheated space.
When to walk away
Walk away when repair, moving and tuning costs exceed the musical value of the piano. This is common with very old upright pianos offered for free.