Could be a great score or a waste of $500. Give us lots of pictures inside and out, and we'll tell you which one it is!
In general, look for sagging metal, splits, cracks in welds, and rust problems.
Make sure the bypass operates and the bypass closes nice and flat, making a good seal. Make sure the door closes flat and isn't warped. Make sure the door glass is in good shape (a little loose is ok, the glass gasket is fairly cheap but the glass isn't).
Expect to drop another $250 for a cat and likely more for gaskets.