SlimRider

First of all, you should NOT buy a used bike online at any time. Buying used from Craigslist requires some type of minimal expertise. This is especially so with mountain bikes. I tend to try to discourage purchasing used aluminum and carbon framed bikes. It would be best to purchase them new do to the fact that you don't know their histories. Chromoly steel bikes that are used, tend to be more forgiving, even after minimal abuse.

I say, go visit many local bicycle shops. Mount and test-ride every bike that you possibly can. Not just mountain bikes!

Volunteer at the nearest bicycle co-op! Make friends there while you volunteer and try to learn basic things like repairing flats, adjusting and installing brakes along with derailleurs.

Ask someone there who's very experienced to tell you how to go about selecting a used bike from Craigslist. They'll most probably either have a frame there on the premises that you can build up yourself, or they'll be able to assist you with your Craigslist search, directly. However, you will have to volunteer at the co-op to get the type of assistance that you're going to need to buy used.

OTOH, if you have a friend or relative who knows bikes, they can accompany you when you go to inspect and purchase a CL bike.

No matter what! You should still join and volunteer at a local bicycle co-op.

Personally, I think you might be happier with a hybrid. New mountain bikes with decent suspension forks, usually cost close to a thousand bucks. If you're riding primarily on paved roads, it wouldn't be too practical to look for a mountain bike. Even buying used will more than likely cost quite a bit.