Admin actually addressed the trading topic once and said:
"The trading options are there, as they came inbuilt with the shop system we bought for the forum, but we cannot enable the trading options if we run the Kpop challenge, because in that case, once a member would collect all the needed cards for the reward, after they get the reward, they could trade the cards they have with a friend (trade all their cards, regardless of their level for lv 1 cards), and in that case their friend would get the rewards with basically 15 lv 1 cards, which would be totally unfair for the members who gained all the cards in a fair way. "
I think this answer is still valid today too.