It depends on the condition, design, brand, etc. If it's a well designed floor standing cast iron jointer like a Jet, Delta, Griz, PM, etc., it could be a decent buy….you can always offer less. If it's a poor quality aluminum benchtop, it's a total ripoff at that price.
Everyone is saying "it depends" and I agree. With older jointers you have to be concerned about how easily you can make adjustments such as moving the fence in or out and changing the angle of the fence. You should also check how easy it is to lower and raise the input table. These are things that often get frozen or gummed up on other jointers.
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