Depends on how "old". Starting early 1960s the manufacturers started putting nylon and plastic gears in sewing machines, and when they wore out, it would cost more to replace the old gears than the machine was worth.
If you are talking pre-1960 machines, they are most likely all steel mechanicals, and yes, they are worth fixing up if not too far gone. A rusted solid boat anchor is not worth it...unless an extremely rare machine...very antique. Anyway, $35 is good for a machine is reasonable working condition..not perfect, but not frozen up. In general, machines are worth what someone will pay for it. International was probably a name put on by one of the major manufacturers for marketing reasons. How old is that one?