Negotiate (CHA)

With Negotiate you can resolve arguments, gather information, and mediate agreements with others.

You can also use Negotiate to get a better deal when bartering with NPCs. As part of negotiating a deal, you may roll a Negotiate check, and the target creature may make a contested Negotiate, 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 an attempt to haggle 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) - this represents the NPC outwitting you (If you're particularly unhappy with the deal, consider theft… or direct violence).