Well, do you want to risk it failing and someone getting hurt or killed?
If you don't mind the risk, get it. If you don't want to be responsible for changing someone's life for the worse, then don't.
Some things are just not worth buying used...
EDIT: Well, it seems you have made up your mind that saving a bit of money is much more important than safety. I imagine that the current owners are metallurgists and engineers so they can just look at something and decide it is OK.
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As long as the rust isn't severe than you should be fine. I would just keep an eye on it over time. You can also slow down the rush with a spray that can be purchased at a local hardware store.
Sounds like it should be fine as long as the springs aren't rusted.