WoW API: PickupMerchantItem
Places the specified merchant item on the cursor.
Interesting thing is this function can be used to drop an item to the merchant as well. This will happen if the cursor already holds an item from player's bag:
PickupContainerItem(bag, slot) PickupMerchantItem(0)
As of patch 2.3 using this function to sell stacks of items does not work, instead it fails silently. Selling unstacked items work, so unstacking and selling items one by one is an alternative.
Blizzard is aware of this issue: http://forums.worldofwarcraft.com/thread.html?topicId=2855994059#12
- Number - The index of the item in the merchant's inventory.