I'm a road cyclist and looking to try out the trail. But, I want to buy a used bike and try to find something under $200. I just want to try it a few times. I started road cycling on a $60 80's something puegeot with a broken drive train and fell in love. So if mountain biking floats my boat as well, then a $200 bike should be good to try it out with.

Anyway, I'm going to search CL, thrift stores, and yard sales. It doesn't help I know nothing about mountain bikes, but I didn't know anything about road bikes either What should I be looking for in a used bike? Are there certain brands, models, components I should aim to get for that price range? Or does it not matter and anything that's at least not a walmart brand bike, will be ok? Is a walmart bike ok? Thanks for any advice!

If you're willing to spend $200, don't get a walmart bike. Almost any used quality bike will be far better. I'd try to find something new-ish, maybe made in the last ten years or so. This will probably get you front suspension (which I suggest.) You can find nice stuff for $200, just post it up here and ask if it's any good. Searching www.bikepedia.com for the bike you're looking at is great because it'll tell you all the specs, MSRP, etc. Good luck!

I know you're in Colorado, but for example, this one looks pretty good: https://madison.craigslist.org/bik/1980353794.html

Thanks for the info, much appreciated. how about something like this?
https://denver.craigslist.org/bik/1985103238.html

I am partial to Giant as well, both my road bikes are Giants and have held up really well.

Or this?
https://denver.craigslist.org/bik/1983604996.html

might be a kid's bike?

I like the Giant ad; the seller indicates hubs and headset serviced, which suggests good care for the bike. Whatever you get, make sure you're happy with the fit and feel of the bike.
Sveikas

Considering one of those is a 17" and the other is a 24 (with a 10.5" ST), it looks like you're not sure what size you need. Best to go to a bike shop and try to get fitted. It will cost a little more, but a cheap bike that doesn't fit isn't a good deal either.

FWIW, the Denver/Boulder/FoCo Craigslists offer a lot better than those two bikes you've listed. A lot of people overprice stuff, so you've got to watch out for that, but lots of nice bikes come up if you're patient. Just figure out what size you need first!

Is there a mountain bike sizing chart?

From Santacruz

Depends on the bike and your riding style. Just like with a road bike, seat tube length is only part of the equation. Reach is an important consideration (and is dependant on the head tube and seat tube angles as well as the top tube length). Some companies have a rider sizing chart for each of their bikes on their website (Yeti and Transition for example). Eventually you'll get a feel for what bikes will probably fit you and what bikes won't by looking at the geometry, but for now, the best you can do is take a guess based on your height and inseam, and see how different bikes feel. How tall are you anyway?