Buying used refrigeration equipment can be really risky. About a year ago I bought a big stainless restaurant-style fridge from a community association (should have been a trustworthy vendor), but only because my brewing buddy is a refrigeration pro that will fix my stuff for nothing. Sure enough, the thing was basically out of refrigerant. It's got a slow leak somewhere; began having trouble with it again earlier this summer and had to recharge the gas. Everyone will say "Oh, we'll plug it in for you and show you that it cools." But there's more to it than that--it may have enough gas psi to get cold, but not enough to keep the condenser from freezing up once it's under some load. Now, you might buy something used that's works flawlessly and gives you year after year of reliable performance. But for every time that happens, there will be 20 times the thing is a piece of junk.
Whether the cooler you are looking at has an "industrial" compressor will depend on the make. My other big cooler is a Coldstream freezer (temp control keeps it functioning as a fridge) with a compressor nearly the size of a WWII bomber engine. Paid $150 (again, knowing service would be free), it holds pressure well, and will probably run as long as I'm brewing.
Now, buying a used chest freezer for dirt cheap off Craigslist or whatever--different can of worms. Residential use, usually really really cheap, might get years of use out of it for almost nothing. I wouldn't hesitate to do it.
Overall, if you have the space then an industrial fridge is a really nice thing to have, but you have to try to find one from a reputable source. Around here the Pepsi distributor frequently sells used coolers. Or find a place that retails or leases them and see if they have something decent. In other words, the more sure you are about the trustworthiness of the vendor, the more likely you are to get something that will not give you headaches down the road.
Good luck!
