A thorough inspection and knowledge of OWB's and about the model in question and I wouldn't hesitate. Checking sediment if any on bottom and using an inspection camera inside the jacket . Looking at the rest of the iron on the property /yard to see how they take care of the rest of their valuables. Pressure or water test at operating temp, warp metal areas. Getting a very good price since your taking the risk.
Sediment is the big one for me, it sludges up and takes a pretty good jet of water to flush out. A simple draining doesn't get it for the ones having problems. If any part of the deal isn't right on, there's always more to look at. I don't regret missing one cuz the price was too high, most times a better deal comes along. If you don't have the knowledge or mechanical ability , it is cheaper to just buy new unless you get lucky. And I wouldn't recommend gambling with this size investment. Good luck either way