I agree about buying a used carbon bike. I would never do it. I do have a Scott Addict bike that is CF but it was well under $3000 and it isn't even my favorite bike. My favorite bikes are made out of steel. Buying a good steel bike now, unfortunately, is a very expensive preposition. Most need to be custom made. There are still some good ones made by All-City, Surly and a few others.
Riding a used Carbon bike would give me the willies, especially when going downhill at about 50 mph. I just wouldn't trust it. CF doesn't fatigue but if you're buying it used, you just don't now what it has been through, if it has been wrecked, clamped, dropped, or whatever. I'm even a little leery of bused aluminum since aluminum does fatigue. Still, though, it's probably safer than CF.
One thing I would add for anyone looking at buying a used bike. Buy a chain wear indicator if you don't already have one. When you look at the prospective bike, take the tool with you and check to see if the chain needs to be replaced. If it does, chance are pretty good that the bike has not been well-maintained regardless of how clean it looks. The problem with riding a bike with a worn chain is that other components like the rear cluster and the chain rings may also be worn. I probably would not buy a bike with a worn chain unless I only wanted it for the frame.
The best used bike I ever bought was someone selling a Schwinn Paramount on Facebook Marketplace for $50. The frame was really nice. It was when Waterford still made them. Them bike was dirty and not maintained but it had that Waterford frame made out of nice Reynolds tubing. I gladly paid $50 and would have paid more. I just wanted the frame. If someone know what they're looking for sometimes there are some gems out there.
Here is that chain checker tool. You can get one for $10-15. Some with more features are maybe close to $30 but something like this is adequate. Everyone that rides should own this and get a new chain when you need to.