Bikes come in sizes. Size affects your comfort, efficiency and performance every single minute that you're riding on it. In terms of importance, nothing else even comes close. You wouldn't buy shoes that were the wrong size just because they were cheap.

When you buy new from a shop you can decide on the kind of bike and price point you want and let the shop get one for you in the right size. When you buy a bike from Craigslist there's usually only one. If it's not the right size it's not a good deal regardless of the price and what components it may have on it.