I haven't been scammed ever on GOG. The GOG community has a system for reporting scammers (scam thread, posts here, standard of lowest rep member going first, etc) so there is some protection.
I can understand how GOG would not encourage trading because it's less likely to be relevent to GOG directly and given negative energy surrounding fear of scams could generate poor will towards the forum if things go awry.
I haven't been scammed here, but I have been scammed elsewhere.
Lowest rep goes first does nothing but protect the person with the higher rep at the expense
of the person with the lower rep. It does even less on GOG since rep can be earned/removed for things completely unrelated to trading. On GOG high rep can mean as little as you gave away a couple cheap bundle games and low rep can mean you've you post things people don't like to hear.
As for "calling out scammers" posts, I've seen them abused more often than not. People posting crap about people they've never traded with because they seem 'suspicious' to them. The rest never seem to offer proof.
Still, banning trading is like throwing the baby out with the bathwater.
(I am getting sick of being asked "what do you want for it" when I specifically asked for offers (but that's Steamtrades for you).