Go to a bike shop and explain that you are looking for an MTB and what do they have. Get an idea of fit either from the LBS or at least by sitting on a variety of frame sizes. Then find out what their cheapest reputable brand is, and maybe lay-by it. You'd be much better off.
Personally, I wouldn't consider an MTB at that price. Probably old, probably in less than good condition, probably needs mucho work to get back into shape. People who advertise their bikes also have a fairly overinflated sense of their market value.