I'll note that the picture of that Trek shows a 1995 Trek 850 with tires on it that match the stock tires for that model. I'm not saying those are the original tires, but it might be a bike that just sat in a garage for the last 20 years.
For $50 I'd take a punt. Worse comes to worse you can WD40 the rust off it and sell it on CL for a profit.