For me, condition is everything. Cosmetic appearance is one thing, but if the machine does not work, IMHO it has little value past being a door stop. That said, I often take non-working machines and restore them, even if it means re-wiring, and more beyond a general service. Bottom line, a working machine is worth more than one that does not.
I once had a fellow try to tell me that a filthy, and I mean covered with dirt, machine worked because he could force the wheel to make the needle rise. I guess he did not notice that the wires had been eaten by a critter, the bobbin case was missing, and we won't even get into the motor! If that happens, go ahead and make a bet that if the seller can get the machine to work right then and there, you'll pay what he asks, and if not, you get the machine for free. That is, if you'd want it.