TL;DR I started http://cultcars.us to find classic cars for sale across all of Facebook Marketplace (and other sites like AutoTrader Classic and eBay)
13 years ago (!) on these forums, I shared a site to search for classic cars across Craigslist and other various popular classic car sites. Now that Craigslist charges a fee to list automotive classified ads, many people have moved onto Facebook for listing their car. Facebook is not a great site for finding what you are looking for, especially when it is outside of your home area. So I created a site to find classic cars across all of Facebook Marketplace (among other sites).
...and if you have any suggestions please let me know! Trying to make something useful.