Your inseam doesn't determine the frame size you need; test rides do. It's a good rule of thumb, but different companies have subtly different ways of measuring and rounding the top-tube length. And different bike geometry can have a big effect ... or it can not.
Buying on Craigslist is a good idea, in general, as long as you shop around. Lots of CL people are going to list it for what they paid, or more. But if you have patience, you'll wind up getting a better deal than you can at the shop. I'd probably avoid used carbon, though. What's your budget?