As my kids were growing up, it was easy to buy used bikes. I'd go to Craigslist, find a bike, and buy it. Was just as easy to sell their old bikes that way. Now there's offerup, letgo, facebook, etc . . .
The last time I bought used, my son was small (just over 5') and a used 46cm Surly CC basically fell out of the sky on Craigslist for $200. Screamer of a deal. Well . . . that kid went and grew into a firmly medium-size person (about 5'8"). He needs a bike for a medium-size person, and I don't think we'll get so lucky as to have this happen again. He LOVES his Crosscheck . . . rides around town, rides to school, rides 30-40 miles with us most weekends, does 50-60 milers when charity events present. New CC is out of his price range, so a good quality used road/gravel bike is in order. He shows no interest in the 80s vintage touring bikes that are so plentiful . . .
Where else to look? offerup hasn't been effective -- lots of bum leads and several bikes that I'm firmly convinced are stolen. Facebook classifieds has some decent bikes. Prices are a little higher there, but that's okay because they're reliable. Just curious where else people look for good used bikes. As I've told him, just watch and wait . . . and be ready to jump at the one you want.
He doesn't want a BD bike . . . mostly because he finds their website beyond awful.
Craigslist has declined in recent years but it's also very location-dependent.
Facebook Marketplace or local buy/sell groups seem to have good options right now.
There's a Salsa and Surly Trader facebook group that has some good options. Crosschecks seem to go pretty quickly there but sometimes people put up a ISO post and get some good responses.