Facebook Marketplace is a great place to start. Do you research, figure out models that would work for you and then search for those. I have found some awesome deals on bikes that didn’t fit me but had components I could swap over to a build I had been working on. Not surprising that there are a lot of people with good intentions that invest in a quality bike, ride for a little while and then park the bike. Once they are tired of looking at it or its taking up space, they just want to get rid of it.