My experience with trying to sell on FB Marketplace is the same as you. Items rejected for no discernable reason e.g. cookbooks, craft stamps, pocketwatches, and there are never any answers to appeals. I have totally given up trying.

GT is no better as most things I list there get plenty of views, but any responses always ask if I will accept a price about 50% of what I have asked. One item I listed for $10 (an absolute bargain) someone asked if I would give it away for free!!

Ebay isn't much better although things have picked up a little this month, but on one of my two IDs I sold a grand total of 4 items for the whole of June (usually sell around 15-20). Other ID wasn't much better.