Hi,

A good local bike store might trump craigslist. One unusual upside of being a larger person is that the low end bikes are stronger and less expensive. A Giant Sedona might be the ticket -
http://www.giant-bicycles.com/en-US/...st/7357/44083/

Or some of the other models at the low end of Giant or Trek or Specialized. A new bike - with warranty and the support of the local bike store.

A local bike store is like a doctor or dentist - it might take a couple of tries to get one you wish to stay with long term. The bike purchase is a good way to open up the vault of service and moral support.

I drop by the bike store that I use a few times a week to shoot the breeze and find it oddly motivating.

As for weight loss - I suggest not worrying about losing weight as a goal. Rather, focus on making one change in your life. Do not worry about the weight but just concentrate on using the bike as your main transport.