Don't be afraid to go to your local bike shop and ask for thier advice too. Just 'cause you're not buying a new bike from them does not mean you're not a customer. They can help you with size questions and maybe steer you in the right direction...
At our shop we have a website where we encourage our customers to post their old "treasures" for sale since we do not have a shop big enough to have used bikes on site. There is a huge demand in my area for used bikes because of the college scene - we have at least 3 campuses within 10 miles of my shop.
Hey! Why not check out the bulletin boards around your local college and also coffee shops near there and telephone poles also get ads stapled to them around here...you won't know what you're looking at until you go look at it. Ride it, see if it is even close to being the right size for you by trying it out. If you think it might be a contender, ask to take it to the local shop for inspection.
There may even be some local bike clubs in your area that might have classifieds sections in their newslewtters or on their website. Ask you shop guys about clubs that can help you with inexpensive bikes.
Otherwise you can search your local dept.of transportation's website and see if they list bike clubs in the pedestrian/cycling section. In NC the dot's site has info on clubs and bike shops across the state as well as free maps to help you tour the state on bikes...
Again, good luck!
<edit> good decision with the fishy guy. If you have to buy it right now, beware.