1977-78 Fender Mustang Bass

So the music store I work in just bought a Fender Mustang bass off an older lady that wanted to get rid of it. She had it laying around in a closet for years and really had no idea what she owned.

From the serial number, I've deduced it was made in 1977 or 1978. Anyway to pinpoint a date?

It's a natural color/black pickguard American made Mustang and comes with the original case even(I think).

It plays great. I really like how the strings have less tension on them. Anywho... What are these things selling for nowdays? I'm looking at just buying it off the store.