In the trade screen the amount to merchant (or NPC) is prepared to pay is listed under each item on your item sidebar. If the that number is in gold text it means you have it equipped at the moment. Once you have selected the items you want to sell you can either drag items you want from the trader in exchange or just ask for gold (by clicking the balance button on their side). If what you want from them is worth more than what you are selling you have to click the balance button on your side.
To repair an item you hit the skills button in the central tool bar bar then arm the repair skill by selecting it from the list now displayed bottom right of screen. Once armed you do a right click and this turns the cursor into a little hammer and anvil. With this cursor active you left click an item you want to repair, either in inventory panel or on you character equipment panel if currently equipped. To repair a second item you have to right click again to get the repair cursor back.
You may find it useful to use the hotkey system for skills and spells. When you open the skill tree panel (by hitting the skill button as I just described) you can hover your cursor over a skill you know and hit an F key. You will see that F key marked on the skill icon as a result of this. Hitting this F key will then arm that skill immediately in future. You can use exactly the same technique for selecting weapons, in this case by hitting the current weapon icon in the central toolbar to open your weapon list. And you can also do this with potions which is incredibly useful - having a heal potion assigned to an F key means you can spam-chug them whilst beating down strong enemies you couldn't otherwise defeat easily.
