It is important to note that you can only go through a negotiation phase with the same merchant once per day. Negotiation requires time and eloquence, and merchants do not appreciate customers insisting repeatedly and may eventually show you the door.
Negotiation phases involve multiple successive tests for each discounted item. The tests stop at the first failure.
Use your negotiation skills wisely, as each interaction with a merchant can have repercussions on your relationship with them, be it positive or negative.