Welcome Ryalor!
1 & 3: Merchants replenish their stock once in a while. It is based on real-time, not game time, so you can't just sleep lots to change their stock. They tend to keep unique and quest items so that you can't lose them in this fashion.
2: If you have a merchant repair all your goods before you sell them, you'll lose profits. Just repair your own equipment and sell your loot in what ever condition it's in. If you have the Repair skill, on the other hand, repair your loot before selling it for additional profit.
