I think the answer heavily depends upon what type of a horse you have for sale and where you’re located.
I have a hunter for sale in the mid-high 5 figures and have him advertised on Exchange Hunter/Jumper, Warmblood-Sales, COTH classifieds, and several FB groups. Almost all of my activity has come from the side marketing that Amber at Exchange HJ does on her FB page. And then a couple of folks have come over from the Thermal Classifieds FB page. I don’t think I’ve gotten anything from Warmblood-Sales, and the one person I got from COTH classifieds was surprised to learn that I’m in the PNW and didn’t want to look at a horse that far away (she was in FL).
So my answer is that if I were advertising a horse for <$25K I’d probably focus on (relatively) local FB pages (up here we’ve got a couple of NW FB pages and then also the Thermal Classifieds). For anything above that I think I’ll stick with the Exchange from this point forward.