Depends on the model. The small B3 may be worth quite a bit but the bigger
models may only be worth $50. At one time a B3 in good condition was very
sought after and with a Leslie resale at a dealer could be over $2000. A friend
of mine paid $50 at a tag sale for a huge 2 manual Hammond with full pedal
board and bench.
It had a problem but I suspect if it were collectable it would have sold and
not been a tag sale leftover. A wonderful instrument but very heavy and very
big. Seems that most people are happy with a small "synthetic" keyboard these
days.
There is a Bluebook for organs but it may not reflect the real going price. Try
calling a few dealers and ask what your model number would cost if they had one
because you were thinking of buying one.