The first thing I would make sure the seller can show you that it works by actually welding something with it. I would also inspect the roller feed and feed tension adjuster. If it looks worn, then the machine as a lot of hours on it. Main thing is that it's not an antique, it looks fairly clean, and the seller can show you it functioning fine. If you can, buy the bottle from the seller too, if it's available. Bottles are surprisingly expensive.
If you run across one of these:
Don't be afraid to buy it. They are big and bulky, but they are good old reliable machines. I have an Idealarc SP200 set up in my fab shop as a stationary welder, since it's kinda heavy and bulky. In my big shop I keep my Ironman 250 in there since it's much smaller and easier to move around when building something big like a trailer.