Buying a used lift worth the savings?

I would say "maybe". If you can actually inspect the OPERATING lifts, up to and including putting a vehicle on them and raise them up and let them back down for several cycles, then yes, I MIGHT would buy one that way. Even still, remember you're spending 500 bucks on something that normally can cost thousands, PLUS, you're relying on it with yours and possibly others lives.
If they are commercial, they will be inspected on a regular basis......At least they are around here. Look at the inspection sticker, might give you some insight.