From a buyers perspective, I see Facebook marketplace as a site where you buy things from people who are cluless of its true value. Ebay is still the safest place to buy high ticket used items. Paid $900 for a NIB GPU during the mining craze in 2020. Ive also purchased some $1000+ used audio gear on ebay as well without issue. Im always skeptical of face to face transactions when prices go over $300, as meeting spots get tricky. .
I've attempted to sell stuff on Facebook Marketplace at or even slightly below fair value and it just sits on the site sometimes for months. That in many cases even includes shipping.
I've sold a shyt ton of items through Facebook Marketplace over the past year and change. I notice FB buyers are usually VERY cheap and want things for bargains. I tend to get items to resale way below retail so I tend to sell my stuff locally for about 30~50% off at times. I've definitely gotten good sales on there. I use Ebay as a second option if the item isn't too heavy as shipping can burn into the cost.
Facebook is totally a buyer's market. A place to dump stuff you just want to get rid of and not have to have the hassle of returns. I offer a 30 day return policy where the buyer pays shipping on everything I sell on eBay just to not pigeon hole buyers into making a money back guarantee claim. On Facebook all sales are final but I end up taking less because like you said the buyers are cheap and they aren't looking to pay close to fair value.