I've been lucky in the few deals I've done with folk on Facebook.
I can only recall two issues here, but rather than make a fuss I learned my lesson and moved on.
Two - out of all the deals I've done and people I've met. Think of the Basschat relays, of a certain member driving miles to deliver a bass and flat out refusing petrol money, the guy who would not take my money until I received and liked the bass, the people who have sold instruments for below the market value and done it happily. People who help me pitch at the right selling price when I have no clue, and all those who take time to leave positive feed back.
Two disappointing moments among years and years of happy buying and selling. I have literally no complaints.