Yeah I do it all the time.
FB Marketplace and CL are typically overpriced. Dreamers. I think they look at their out-the-door price paid that includes all the mounting etc then divide it by perceived life remaining, but they neglect to account for the last 2/32" that nobody wants to use. And, of course, the mounting fees the buyer will suffer.
I search by tire size but phrase it a couple different ways... 195 65 15, 195/65/15, 195/65R15 etc. The search algorithms are good but not perfect. There are also dopes who will advertise "goodyear tires for cavalier" without mentioning the size, and they probably miss a bigger audience.
I used to have good luck with a disorganized junkyard that charged $5-10 a tire, and this was in 2012 when new tires were $75+ due to an oil crisis. I like buying from a junkyard since the cars rusted out and the tires are probably fine. Tires from a tire store are immediately suspect-- someone decided they were so bad they had to come off before they wore out. They charge more now and new tires are cheaper.
I keep hitting walmart, ebay, and amazon with my tire sizes-- there are occasional flash sales on new rubber. Just got some "Supermax" for my camry for 30-something bucks a tire shipped. Like my oil stash, I try and do a tire stash... buying 3-6 months before I need them.
I'll add that I have a tire mounter and balancer. Sometimes I'll get a set of tires on rims, separate them, and make my money back on the part I didn't want.