I will say this,.... $600 for a lift is dirt cheap, so as DCCoronet said, check it out very closely! I will agree with 493Mike that it is money well spent, and I LOVE mine! However, as my wife said when I was looking at lifts,... "Please do not skimp on something that will be holding a car over your head!!". It's the whole reason I waited until I could afford to buy a new lift from a reputable company.
It is all about how good the lifts were when they were new, and what kind of condition they are currently in. I would be a little nervous about why the dealership took it out of service. I understand they said they change them out on a preventative basis, but lifts aren't cheap. We all know how businesses are,.... they don't spend money on things like that unless it's needed (like a safety concern). There's a difference between getting one from a shop that went out of business, and getting one from a shop that "Chose" to replace it. There had to be a reason they changed them beyond just a schedule.
Please just check it out well and make sure the lock system is strong, cables are good, and there is NO wear on the arms. Like I said,... they are very handy, but working with a car over your head is not a good time for a failure!!