Actually, to be completely on the "up and up", instrument purchases used for business purposes should be depreciated to zero value or a minimal amount, so you can deduct the cost plus interest, if any. Then, when the equipment is sold, you should pay capital gains tax on the difference between the sale price and the depreciated (book) value.
If done correctly, it doesn't make much difference what you sell old, depreciated equipment for.
Collectible guitars are where the values are. I purchased a Gibson L-5 for around $1,000.00
in the early 70's. Just had an offer from a Philharmonic for $17,500.00. I'll donate it, because I'd have to pay capital gains tax on $16,500.00. Over a 50 year period, appreciation on my string instruments is in excess of 23.7% a year.
Man,I wish that were the case with keyboards and other electronic equipment.
IT AIN'T!
Russ