I would say there are risk buying 2nd hand ESC & motor. Unless if you knew the seller well enough. But usually problem happens when water hits the electronics. The seller might dried them off & sell it off immediately.

To my experience I can say this problem comes from ESC. What happens when I connect the ESC to receiver & servo. The fans turn at inconsistant rate. I mean one on the motor & other at eSC if you observe carefully The servo are shaking or somehow rattling as it did not get enough current. Since the ESC is under warrantly from Novak. I had it replaced.

When I swap another new eSC everything works perfectly.