It's not allowed as others have pointed out, there was a big kerfuffle on the art side of the fandom when that was clarified as well back when gil was harder to make. I'd also warn people that if they try to get around it be careful of what they say in game and who they try to jump through hoops for. Most artist friends I have will do it but only for good friends and will never discuss the actual transaction in game given the risk of being reported.