If you give us your height we could give you a rough estimate of what sizes to look for. You might also try a google search for a bicycle fit calculator.

Prices vary by region. Just because the guy is asking $100 doesn't mean he won't accept less. Before going you should do google search on buying used bicycle.

Basically take the bike for a ride to determine fit or adjustment needs. It will also allow you to check brakes, shifting, steering or whatever else. Inspect the bike for dirt, grime, wear, and so on.

Also don't be afraid to ask the guy questions such as; How long have you owned the bike, Where was it bought, Where has it been ridden, Why are you selling it, Has it been in any accidents, etc. If it seems shady then buyer beware.