This is Bakari's guide to buying a bike on craigslist.
+1 That's as close to a "guide" as you'll get.
The hardest part will be knowing what size fits you when you haven't biked in a while. Standard advice suggests visiting a local bike shop to get measured. Be prepared, however, for many CL ads to have zero sizing information and not to respond to questions about size ahead of time.
The beauty of buying a used bike is, unless you dramatically overpay for something, if the bike doesn't suit you after a few months or a year, you can recoup almost all of your money, if not all of if.