Most valuable part of any used bike is the stickers. See all posts above suggesting name brands. Stickers are just stickers. They do sell bikes. Second most valuable part is paint. Skeptical as I might sound I will buy a bike for paint. Most anyone will.
Unless you already know everything Everything! buy a bike that can be test ridden. One that scores as perfect on test ride. Sellers who say no test ride can be forgotten real quick.
One possible exception is garage queens that plainly have never been ridden. Or did 50 miles only many years ago. Now I'll tell you the secret of those. Many of them, probably the majority of them, have one great big original assembly error. Which is why they were never ridden. If you can analyze and identify that error quickly, and before you risk any of your own skin, and if you can repair that error for free with scavenged parts, if you can do all that then unused garage queens are great buys.
Clean looking bikes with desirable stickers that are going to be a nightmare sell for just about same price as exact same bike in perfect operating condition.