Have the seller demonstrate that it powers in and out, the clutch works, and it free spools.
If the seller can't or won't demonstrate that his used winch is in working condition you transition from potential buyer to gambling man. Only you know your risk tolerance and your betting limit.
I recently upgraded my winch and sold my old one. I made sure that all the mounting bolts were free and that it wouldn't take more than a few minutes to disconnect the power cables but waited for the buyer to see for himself that the winch worked as it should before I pulled it from my jeep.