You need a Trader’s Contract to trade. Go to Shopping -> Items to buy it.
There are different levels of rarity for cards.
We have common < uncommon < rare < very rare cards in that order. The rarity can be confirmed by either the symbol on the card or pulling it up in the card list section.
As with any other card game some cards from the same rarity group are in higher demand than others and as such they’re valued differently.
Check each value of the card before you trade it. The big thing to learn when you’re starting is the relative values of each card. If you’re working out a trade and not sure if it’s a fair deal ask one moderator or a high ranked player. You will usually get a straight up answer or at least some input on your pending trade.
Most of the higher ranked players are looking for a specific set of cads. If you just started and are trying to get some of the more common cards for your deck instead of wasting your time with the higher ranked player check with players closer to the starting rating of 1200.
Use the “Trade” section in the forums. It’s a great place to post all your cards for trade and a list of what cards you are looking for. It will give you far more exposure then just posting it in the lobby.
Try to talk face to face with a player you want to trade with. It’s always a lot easier when you can go back and forth on the specifics rather than just offering on their cards in trade.
If you were offered some good cards doesn’t necessarily mean that it’s the best deal out there. You might be able to find someone who will offer even more. Don’t jump on the first offer that seems to look ok but rather give it a day or two to see if anyone will offer anything better.
Very Rares are sought after by every player. If you manage to get your hands on one of those you can expect to get quite a lot for it. If you’re just starting try to trade it for a big selection of commons, uncommons and rares.
Although it’s a lot harder to get one of those than some of the cards you might be getting for it, on the flip side it’s a lot easier to make a deck with a good selection of card rather than just one very rare.