The problem in getting a fair price is how much demand the tool has.
You can find a Sears saw that sold for $500 and have trouble getting $100
for it. A used unisaw would sell for a bigger percentage. I like to check
the newspapers for used prices on things to get an idea of what people are
paying for things.
max

If the machine is in good shape, half of what the machine would cost
new today is a good guide.

Steve