You want to check the liner up inside the jet to see if it smooth or marked up with any deep scratches or grooves. The liner is replaceable but any wear will affect performance. Also look for any damage to the leading edges of the impeller. If you can, it would be nice to measure the impeller clearance but I'm not sure if you can do that with the grates on the foot. You could also check the grease in the main bearing by having the seller pump some new grease in to see what the old grease looks like coming out of the hose. There is a short hose that loops around and that's where the grease fitting is and the bearing inside. If the grates are bent up, they can be removed and straightened out most of the time.
You can see the hose looped in this picture.