Where to sell musical instruments and sheet music?

My dh buys, fixes/sets up, re-sells guitars and banjos. Some he buys from ebay, some from boot fairs etc. He is a guitarist and has worked with them all his life. He knows what he's doing.

He might buy an instrument for, say, 50 quid, set it up properly and fix bits, then he'll sell it for what it's worth, say, 85 quid. He doesn't overcharge when he sells on.

It's not the only string to his bow, luckily! He says it's really hard at the moment as no-one seems to have any spare money to buy an instrument - which is not an essential - and people selling want far more than they're worth. So he's not buying.