Call me stupid, but I think the right thing to do would be to contact the original owner, and give it back to him. It's bad enough times got tough enough he lost his home, and he clearly would not have left this behind without it being a mistake.
Isn't it interesting how honesty is always more complex than it appears? Of course Tim is right: how many of us had not thought the whole situation through and back to first base? Me for one! And why was the bass in the attic in the first place? Was it being stored for someone else again? That could explain why it was forgotten.
On the other hand, if the builder had advertised the bass for $500 for a quick profit, who among us would not have bought it if we had the chance?
Life is only simple once the music starts and the left hand is on the fretboard...that's the only time that I seem to know what's expected of me!!!