These are the items in your backpack. When you are trading you can double-click an item to move it to the "sell" area; items with a red tint indicates items that the person you are trading with is not interested in. When you are outside the trading screen, ie. just looking at your inventory, you can double-click items to perform the default action for them - ie. eat food, move artifacts to an available artifact container, equip a weapon or armor, etc. You can also right-click an item to display the menu, which will show you what actions are available; this is useful for removing weapon attachments, emptying weapons of ammunition, dropping items, and similar. If you hover your mouse cursor over a weapon, you can see if you (or the trader) has any appropriate ammunition for that weapon in your backpack (or his inventory); similarly, you can hover the mouse over ammunition and see which weapons in your backpack, or the traders inventory, use that particular type of ammunition.