I haven't sold on eBay for years, but I do buy things there sometimes. However, after a bad experience last year, I've become extremely careful and buy only from sellers with a substantial amount of feedback.
For me, eBay remains a good place to find discontinued items, like my preferred computer keyboard or a particular style of Jansport backpack.
I'm not on FB, so I've never used Marketplace. When I'm looking for an item of clothing, I also check out Poshmark.
OP, I think it all depends on what you're selling, its condition, its price, and how it's listed. I won't even consider something--even if it's exactly what I was looking for--if there aren't enough or good-enough photos and/or if the description is lacking. Sometimes sellers use stock photos and I won't purchase anything that doesn't have a photo of the actual item being offered.
I sometimes make offers and only sort of lowball when an item is hugely overpriced.
OP, I don't think there's a magic answer to your problem, but before you list something, check out sold listings of the same item and see what it sold for, its condition, etc. If you see a lot of expired, unsold listings for a similar item, that's good information.