The big advantage trading has over everything else we've discussed is that everyone already understands it, and it's been proven to work and be ultimately accepted by the users and profitable to the company. There are two big disadvantages, one is that the game isn't designed for it, and two is the legal implications.
When I'm talking about game design, I'm talking about overall. As people have mentioned, things like give out free cards and free gran would have to be carefully monitored and pre-calculated. Also, the game balance. Now, it's been mentioned that the game balance might not be "good enough" for trading. I'd say that it's more like that game balance would have to be very different in a trading environment. Right now, two environments are taken into consideration when cards are balanced. The first is the all-cards environment, which approximates the highest level of play, and the second is the majority-game environment, which takes into account the number of each card likely to be in circulation and how you get them. For example, many have noticed there are 1 Star "foundation" cards, meaning cards that can be the foundation of a variety of decks, that are not in the Starters. Guardian, for example. This was done on purpose, so that a free player has an obtainable goal that can make a big difference in what he's capable of. And there are lots of other types of cards out there that just wouldn't make sense in a trading system. So, every card would have to be re-blanced with the new system in mind. Also, the number of each rarity, the number of cards in each set, these were all chosen based on the type of system we have. Perhaps the biggest issue would be deck size, which is significantly smaller than in any trading-based game I've ever played. The number of cards needed to build each deck being lower might also have an effect on the stability of the system. I've only been thinking about it a short time and I've already got a list of concerns that would have to be addressed by someone much smarter than myself. I have worked for Upper Deck in the past, and I know when they're figuring out number of cards in a set, number of each rarity, print runs, they put a literal army of mathematicians, statisticians and game theorists on it.
The other issue is legal. Again, I'm no expert but when this whole thing started I had a short meeting with an expert on online law, and basically New York City is a pretty challenging place to run a virtual economy. When users can engage in transactions of their own using virtual goods as if they had real world value, things get heated up. And I'm not just talking about someone accusing someone else of stealing or tricking them into trading away a good card. I'm talking about 2 guys who go watch a duel using the duel spectating feature, each one picks a favorite and throws a 3star up on the trading window. The one who's pick wins takes them both. That's gambling, effectively on our property with tools we've created, and very illegal here.
"Scissors are overpowered. Rock is fine." -Paper