Know what you're buying. Some bikes come from bike shops. Those bikes hold up better over time (and multiple owners). Other bikes- mostly from department/big box stores- don't hold up as well. These brands are department store bikes:
This means they weren't originally assembled by trained bicycle mechanics. They were originally assembled by the guy (I assume) who's also responsible for assembling grills, flat pack furniture, and baby cribs. They have 'entry level' (read: cheap) components, that don't last as long as their 'mid level' counter parts. Many of the components I've seen on these bikes aren't serviceable- so when they go outta whack you're looking at replacement, not adjustment. Bottom line- they may not be 'bad' bikes, per se, but they have a short working lifespan. If you're buying one used it's short life is even shorter. I won't say don't buy one of these bikes, but......... don't buy one of these bikes.
The guy who posted his used bike is 5'10", and so are you, but that doesn't mean his bike is going to fit you. Maybe he's been riding a bike that's too big or too small for him this whole time. Maybe he has oddly short legs, or long arms. Maybe you have oddly long legs and/or arms. Bring a multi tool with you so you can adjust the seat when you take it for a test ride.You were going to test ride it, right?
Test ride it. Make sure there's air in the tires, that the brakes work, and that the quick releases are tight. Put your helmet on. Then take it for a spin. Keep an ear out of creaking, squeaking, clunking, etc. Noises don't mean you shouldn't buy it, but it will be helpful information to pass along to the mechanic when you bring it in for a check over.
Bring it in to your LBS to get it checked out. Even if the posting claims that it's been "recently tuned up". "Recently tuned up" is a relative term that can mean anywhere from 'tuned up at an actual bike shop' to a 'buddy of mine (who knows alot about bikes) looked at it'. At the very least you want a trained set of eyes to make sure it's safe and worth the money.
If you don't want to wait around for your dream bike (in the right size) to pop up on craigslist, you can always check out what the fellas at Cambridge Used Bikes/ Metonomy Used Bicycles or support the good work being done at Bikes Not Bombs by buying a used bike from those folks.