Negotiation (CHA)

With Negotiation you can resolve arguments, gather information, and negotiate agreements with others.

You can also use Negotiation to get a better deal when bartering with NPCs. As part of negotiating a deal, you may roll Diplomacy, and the target creature may make a contested Negotiation, Charm, Con, or Intimidate roll against you. Depending on whether you succeed or fail, you may see changes in their offerings change up to 50% in either direction, either favorably or unfavorably towards you.

If a Haggle check is made then the trade must happen, even if you end up having to sell more then you intended to (or even go into debt). It represents the NPC outwitting you. (If you're really unhappy with the deal consider theft… or direct violence)