Buy
This is the left most button beneath the NPC inventory area in the trade/repair interface.
Sell
This is the second button beneath the NPC inventory area in the trade/repair interface.
Repair
This is the third button beneath the NPC inventory area in the trade/repair interface.
Cancel
This is the right most button beneath the NPC inventory area in the trade/repair interface.
Armor
This is the left most tab above the NPC inventory area in the trade/repair interface.
Weapons
This is the second tab above the NPC inventory area in the trade/repair interface.
Misc
This is the right most tab above the NPC inventory area in the trade/repair interface.