You'll probably pick up on this right away if you spend any time on Popup Times, but another thing to look for with used ones is a sagging roof if it's a Coleman. I *think* the years that had the problem were 1999-2000, but it may have started earlier. It would only apply to models that have the ABS roof.
BTW, I love the ABS roof, since I have one that doesn't sag. It's a wonderful plastic material that isn't noisy in the rain. I can't imagine sleeping in the rain in one with a metal roof. If you like to sleep in the rain, it may not matter, but I don't.