Before you buy play each note - are they clear? do they buzz? Check for woodworm! (Honest) Take the ends off and check how the reeds are attached, if there are problems and the reeds are rivetted to a block (usually about 20 reeds per block) I would reject. If the reeds are individually mounted on seperate blocks they're easier to sort out.

CHECK THE BELLOWS!! Small holes can be cured with electrical tape (my C/G has survived 20 years like that) but larger holes are trouble. If any notes sound without a button being pressed reject it (or offer a fiver)

All the best

Steve