Is there some sort of market place to sell / trade cards you find? I imagine if you find a rare, you can sell it for gold coins. Maybe they will have it at the start and then remove the card AH after a year.
Nope, no trading at all, all cards are "soulbound" and it's really unfortunate.
It's a limit of 2 of each card per deck, or 1 per deck for legendary. You can disenchant your extra cards into dust, and dust can be used to craft cards you need. I'm not sure of all the ratios, but I think it's around 8 cards you need to dust to get enough to craft one of equal rarity.
I understand the anti-smurf account argument, but blizzard is smart enough to come up with a work-around to discourage bot or smurf accounts without completely removing the possibility to ever trade. For example, if cards bought with daily quest/achievement gold exceed 50% of your total cards, you can't trade at all, but if you have actually bought at least half your cards for real money trading is enabled. Something like that would make botting non-viable as a way to make free money, the bots wouldn't be able to trade unless they had real money invested into them. I wish they would reconsider.