Interesting start. I'm not inclined to give the 1 BTC/instrument if that's what you want to sell them for, because it seems an awful lot like $500 and $50 would be a double dip commission. If you want a finder's fee more than 1 BTC/instrument, it should be from the buyers separately.

But, if you have buyers right now who can pay me (violin) and the non-profit (that owns the piano) directly - and they agree to pay shipping and freight charges, if they can't pick them up directly, I can accept each offer individually and on the non-profit's behalf. Please PM me with their details and your BTC address to send the 2 BTC when the sales are final. Thanks.