I doubt they want their forums to become a bartering floor, but there's literally no place else to find people who want to trade Paradox games. There's nothing in the rules against it as far as I've seen.
Its not anywhere near as easy as Steam to trade stuff from GG, so I doubt you're going to see much of it. I've never seen anyone else do it before, but the moderators let me do it for a CK2:SI key I bought by mistake.