In addition to the routine pin wear and signs of leaks, a few other generic things to look at:
1. The condition of the chrome on the cylinder rods - if they are rusting or the chrome is worn, this is a leak waiting to happen (if it hasn't already). Any equipment that we store outside, we typically retract all of the cylinders to teh best of our ability to protect the rods.
2. Signs of cracks or repairs - not the end of the world if they are repaired properly, but should have an influence on the price.
3. The hoses - age and UV light are hard on hydraulic hoses. If the outer covers are cracked, and they are 1-wire hoses (eg. 100R1), they could become an issue sooner than later. Also, look for signs of cover deterioration next to the crimps.
HTH,
SR