SE has not said whether it's allowed or not, and after seeing how fast they reacted to people cheating in T8 and position hacks and whatnot, I dare say if win trading was considered against the ToS they would have already done something about it. A big part of the PvP community will QQ, complain, call people names and threaten to quit the game. SE's reaction so far (or what I like to think their reaction is)