Before you can buy items from someone else, you must send a trade request.
To send such a request you must:
1. Open the action menu (press the left command button while in normal game mode)
2. Select the rightmost button (with a dollar sign on it) using the LEFT/RIGHT keys, then hit FIRE to activate it
3. With the LEFT/RIGHT and UP/DOWN keys you can cycle through all the players visible on your screen
4. After the player of your choice is within the white selection frame, hit FIRE to send the trade request
5. You will then receive a list of items which this character is currently selling
6. Navigate the list like you navigate your inventory and use the FIRE key to instantly buy the item
7. In case you have enough gold to buy the given item, it will be transferred to your inventory and the seller will receive the gold value of the item which will be subtracted from your gold
If you have items for sale, players (including NPC traders) can request a trade offer from you aswell, they will see the items you are selling in such a list as described above.
You set items for sale by using your inventory context menu for the given item. See inventory.