people on the boston craigslist (as in every major metropolitan area) want way more money for their crappy old road bikes than said crappy road bikes are worth. fortunately, most of them can be talked down pretty easily.

here is my advice:

first, make sure whatever bike you are checking out FITS you. measure yourself and read sheldon brown to figure out a good size range.

also: be positive that the bike you are checking out has horizontal dropouts. again, if you don't know what that means, read sheldon brown.

the t has a fairly good commuter rail system that will take you basically an hour outside the city in pretty much any direction. if you want to get a good deal, GO TO THE SUBURBS! i'm not even kidding about this; bikes that go (very quickly) INSIDE the city limits for upwards of 100 dollars go for 30-50 dollars + a 10 dollar round trip on the commuter rail. and most people who are selling an old road bike in the suburbs don't know/don't care about their bikes - they are very often just trying to clean out their basements and make a quick buck - THEIR PRICES CAN ALMOST ALWAYS BE TALKED DOWN - trust me on this one.