The most important thing is whether the bike fits you. If you don't know what size the bike you are looking at is, or what size you reqire - you should determine this before buying any bike. An inexpensive bike that fits will serve you better than an expensive bike that doesn't.

If it fits, is in good shape , and you like it - buy it and get biking. I don't reallly know the market for used bikes but heck you're talking $175 for a working bike in a world where just two tires (gaterskin) can set you back over $90.