there is totally fees and sales tax on facebook marketplace when you purchase or sell THROUGH facebook. i.e. press the "buy it now" button.
If you message someone, and meet up to do a local sale, of course there isnt fees, unless you tell Facebook that you did that.
When you mark an item as sold it asks you if you sold it on facebook, or elsewhere. if you select facebook, it takes a cut - even for local.
But, thinking facebook doesnt collect taxes or take out a cut for using their service, is nonsense.