It's a Trek 6700. From a little searching, I think it's an 08 model. Says it has upgraded brakes. Asking $650.

The bike is a 15.5" frame, and I'm ~5'7". I think that's a good fit, but the ad said good for someone 5'9"-5'10". What do you guys think? It also said it's a good bike for someone weighing less that 140lbs. Sounds off to me, but maybe I'm crazy? I wouldn't think there are many who are 5'10" and weigh under 140lbs anyway. I'm 3" less, 10lbs more, and I'm fairly thin.

Does this sound like a decent first bike at a decent price?

Or should I perhaps stick to a new bike?

I think a 15.5 frame is what you need. The big test is if you can straddle the top tube and have a couple inches of "fudge factor."

As for price, it might be a bit steep. If the bike is in primo shape, $650. can be a fair price. $500. sounds more reasonable for a now 4 year old model. It looks like a quality ride with good basic components. That is just how I see it.

Daspydyr is correct on all points, good luck!

agreed on all points.
What I've learned the hard way with MTB's used is that there is a lot more that could need work. Shocks, wheels etc.
Good Luck!