Get knowledge, THEN shop.
Get knowledge =
Step 1A: google bike sizing, take some measurements, estimate your size, test ride a few bikes, and target that size.
Step 1B: Calibrate your vision, so from a craptastic picture you can estimate size. Sellers rarely/never get size right in my experience.
Step 2: Spend some time on google, learning how to inspect a used bike, what to look for, and values. With the web, its not as hard as it sounds. Many have asked this question before, and gotten hundreds if not thousands of opinions on this topic. Google will get you more information than you ever could imagine.
Step 3: Be ready to pounce when a deal pops up. If it is a deal, the seller will get inundated with responses. Keep yours SIMPLE, or you will go to the bottom of the list. There is no "dibs" on C/L. Sellers can and WILL respond to buyers out of order, based on how easy it sounds to complete the deal. I sold a bike this week. The first response I got was from someone that sent me several emails, asking for the exact weight of the bike, a detailed listing of every component, age of everything, etc. They even had a friend of theirs send me an email asking why I hadn't responded. I had five other responses. Responder
#6 was the most straight forward: "I would like your bike, when is a good time to meet?"
Guess who I called back???
For some reason, some think there is some kind of dibs list, "I sent a message first, so I have dibs on the bike." Dibs went out decades ago.
The good news is that Trek was a can of worms. Even if it had fit, it was best to avoid it. So no loss on that one.