If you're in the market for a GMT400, Facebook Marketplace would probably be your best bet. I tried numerous used car sites — because I did not have a Facebook account when I began my search — and it was just painful. (Word of advice: do NOT use Autotrader.) Probably within a day or so of creating a new Facebook account (I had one in the past but deleted it), I found my GMT400 in Arkansas and contacted the owner immediately. It took some coordination on my part and patience on the owner's part, but it was a success.
Be mindful that there are scammers out there, so sellers might not be as responsive as you like, because they assume you're just another scammer. Be persistent to demonstrate you're sincerely interested.
Be willing to travel to buy it. You probably don't want to buy one that has lived its life in the rust belt.
Inspect it or have someone inspect it (there are companies out there that perform used car inspections on your behalf) before buying it. Do NOT buy it without first inspecting it.
Be ready to make a deal fast when you find one. They go quick. Cash4Clunkers really depleted the market of GMT400s. It's a sad reality.
By the way, the 4th gen Silverados are sweet! I'm in love with the 2019+ Silverados that have the 10L80 10-speed transmission. I'll probably wait about 10 years to get a used one myself.
