I think that you should be allowed to sell on the market after you buy something with credits. Buying a pack with credits earned from tourneys only take a long enough time that there wouldn't be a huge amount of people cheating using multi accounts.
I dont get the whole cheating thing....
If you make a new account then you can trade your good cards with another of your account => this is cheating
Working hard for your credits (you have to place yourself in the first half of all the people during the tournament) and they spend them on a new deck => not cheating
Maybe i forgot something.....
We usally run promotions with free credits (or poll, events.. whatever). We cannot distinguish "earn credits" (by
playing) from "free credits" (by promo).
Anyway, it's a 2 euros thing for a lifetime, if you really like the game and if you want to sell your cards, it's not much
to ask for (I think).