Bicycle type for the obese

Your weight is not THAT high that a quality bike won't be able to handle it. Try a local bike shop and see what they recommend. You may even be able to score something used, from trade-ins, or check your local paper for used ones.

We have 4 bikes in our garage, and three are hybrids. They tend to have beefier frames, and wider, more knobby tires. They aren't too heavy to do some road riding with, but are sturdier for trail riding. If you know you are going to be trail riding primarily, I'd look at mountain bikes that are even sturdier. They tend to get beat up, flying down hills and such!

If by "trails" you mean urban/suburban multiuse paths, then a hybrid is probably the way to go. They look similar to mountain bikes, but with slight geometry changes to make a more comfortable seating position. They also have more road-ish tires; ie, tires well adapted for smoother surfaces. Usually a hybrid can handle an average or better-than-average gravel road, but not well suited for offroad use.

Mountain bikes are similar, but with a somewhat more aggressive body position and - biggest difference - larger, knobbier tires. These will work much better in sand, dirt, or mud - but the tradeoff is that they are a lot noisier and draggier on paved roads. You can still use them on pavement, but you'll be working harder to go the same speed (or go slower with the same level of effort).

Craigslist CAN be a good way to pick up a bike at good value - a lot of people buy bikes and never ride them, then sell them later on craigslist. But I would at least pay a visit to a local bike store. Your budget would allow you to pick up a very decent hybrid or MTB at a bike store. The advice and guidance you'd get at a bike store could easily be worth whatever money you'd save on Craigslist. Also, I've noticed lately from following Craigslist ads that people seem to have a really inflated idea of what used bikes are worth, and in some cases they're not discounted very much from what you'd pay at a store (also Springtime is not the best season for discount bike shopping)

A few tips on riding comfort:
1) Give yourself some time to toughen up your backside. You will almost certainly have a sore butt. Take it easy, give yourself some time to build up. Be sure to take a walk after riding - much better butt therapy than just going inside and sitting down.
2) Real bike pants with a built in chamois DO help. If you're too self conscious to do the spandex thing, look at mountain bike shorts which have a baggy outside layer and real bike shorts as an inner layer. Or just get a pair of real bike shorts and wear them as "underwear" under a looser fitting pair of shorts.
3) Avoid the temptation to get a huge, puffy-looking sofa style bike seat. It looks comfortable, but really it is just a bunch of extra material that gets in the way and digs into places you don't want dug into.
4) It really helps to get the RIGHT SIZE BIKE and have it adjusted properly for you. This is a HUGE PLUS of going to a real bike store.

One more thing: whether you go to a bike store or wind up doing Craigslist, DO DEFINITELY stick with the brands that are sold in real bike stores. Specialized, Cannondale, Giant, Fuji, Bianchi, Trek. Do not fool around with the "department store" bike brands. They will give you nothing but trouble.

Yay! Have fun! I've ridden three days in a row, my butt's a little tender!