for the 5xx models, Check the brushes. Spin them with your fingers, if they don't spin with ease, could indicate stuffed up gears. But it if was only used 4 times (I never believe those stories), I doubt that would be a problem. Also, ask him to charge the unit up and make sure you see it run. Paying $200 for a USED roomba , you want to make sure everything works.
I got my 551 which was barely used on Craigslist for $75.00.
Also, consider Ebay
. You can grab a deal on "open box - never used" 4xx & 4xxx series under $100.00.
If you are going to be running this during the night, maybe the 5xx is a better idea as it is much quieter.
So to answer your question :
Battery life, Make sure brushes spin freely, and see the unit run before buying it. (if unfamiliar with Roombas, check youtube to see how a "normal" roomba operates.