I mainly put non-evangelical and non-religious things on eBay now, as well as some Christian books (whether Reformed or broadly evangelical) that aren't selling on FB. Nice Bibles usually sell quickly on FB. But sometimes more run of the mill Christian books don't, depending on who sees them and what they are. I think that some of the FB groups are so big now and some of them have so many posts that some of the posts probably get overlooked. I know that I don't go through and check every post except for the group that I moderate. I've missed out on some great deals that way, but I don't have time to monitor dozens if not 100+ posts per day in maybe 6-10 different groups.
As far as I know, you can't set up automated searches that will notify you when something you want is posted the way you can on eBay, Abebooks, and maybe a few other sites like that. From a buyer standpoint, that's one thing that eBay still has going for it vs Amazon.
Even with Bibles, I was surprised that green NKJV Quentel that you bought a while back in one of the FB groups ended up being available for so long, for example. (I think you're the one who bought it.) Maybe it was the color, which isn't everybody's cup of tea. I was tempted, but I had bought a black one around that time in addition to a calfskin I already had.