I once saw a charity shop (the British Heart Foundation in Nicolson Street, Edinburgh, to be precise) with a tuba in their window priced at 60 pounds. In excellent condition, with a case and mouthpiece. So I went in, told them they had underpriced it by a factor of about 20 at least, and went on to put the word round the local brass band scene so they could negotiate a fair price.

Came back from holiday a couple of weeks later to find out what had happened.   Yes they'd sold it to a band. Ok, what did they get for it? The manageress of the shop flatly refused to tell me.

That was the last time I offered them the least bit of helpful advice. Got a first edition of "Ulysses" for two quid? okay I'll take it.