buying lens from craigslist...advice requested

I've just moved up to FF and need to change my lens lineup. Given I just bought the FF body, I am a little cash strapped right now so I've been looking at craigslist to get me by while I save up for my dream lens.

Given I am a nautrally skeptical person, I am only looking at $200 and under lenses and have zeroed in on the 50mm 1.8D or 50mm 1.8G lens. Is there anything in particular that I should be looking at when testing these lenses?

I tried to search the forums but the list of threads that comes up when I use "craigslist" or "buying used" is long and after a couple of pages of forums I couldn't find any advice. Sorry if this has been already discussed (probably has).

Thanks in advance.