According to bicycle bluebook.com, the resale value ranges from $137 to $171 (depending on whether it's a 2006 or 2007 model, and depending on the condition). Since the seller hasn't provided a picture of the bicycle he's actually selling, we can't really gauge the condition for ourselves. However, he's also throwing in a few extras (seat upgrade, pump, cyclocomputer, a U-lock, and a helmet). Even assuming that these are all very cheap, low-end components, they add up and make the $200 asking price anywhere from reasonable to a good deal (depending on the condition of the bike). It's also worth noting that the bicycle bluebook seems to give values that are always below what's listed on Craigslist; if you can find a listing that is under $100 over what the bluebook suggests, I consider that to be a decent deal.

Regarding frame size, you can use the eBicycles frame calculator to determine what sizes fit for you. Based on the photo, the Bianchi boardwalk is a hybrid using a road bike frame, so use the calculator for road bikes.