OK, in the stores when you are selling to a vendor, when you move your cursor over the item you are selling, it will show the price that the item would sell to an NPC (non player character). This price is NOT what you receive - you get the price that the vendor has set. This system is a check for you to see if the vendor is selling for more than the NPC, in other words a way to make sure you are getting a good deal.
Now, when trading, it happens in a certain sequence. You and the other person will place the items and/or cash to be traded in the squares. Next, each of you has a small button to the left to lock in your trade. Each player HAS to click this button to lock in the trade. Finally, each player then has to click the Approve button. This action will complete the trade. Now, if you lock in your side of the trade and the other player then changes something on his side, your side unlocks as a safety measure so that you don't get tricked into something you did not agree to. If this happens and still want to trade, lock your side again and then finish out the trade.
I hope this helps. Also, feel free to click the Wiki button on the top of the page to learn more about the game.