Well the trading mechanic is simple: Trade can only be done directly. You initiate the trade by clicking on the trade button on the wall of the person you want to trade with. Then you select up to 5 items in your trade offer and send. That person you've sent trade to will then check your offer in his trade section. He can cancel the trade if he doesn't like it or if he accepts it he will send the item(s). Then you, the person who initiates the trade, get to finalize the trade. After you click that accept button, there's no going back. There's no return policy in this game or if you get scammed you won't get support from mobage. So do trade at your own risk. You can also change your mind and cancel the trade at the last minute like the big jerk you are, lol jk :p
As for card's worth, yea generally the more * it has the more valuable it is. There are exceptions, as some 4*, 5*, or 6* are really really rare or has really nice art or both, those factors can boost their value. You should check this site for a general idea of how much a card is worth. You shouldn't completely trust that site but the price on it can be quite close to the true value. What I do is I check both how much ppl are selling and buying it with, then I price it somewhere in the middle.