It's tricky roleplaying bargaining in a game. At least in the real world you actually have some sort of idea about the values of the items you're trading. (Or you should, or what are you doing bargaining in the first place? Trust us on this one.)
But in a game you're often doing things like trying to get a better price in silver nobles for a magic sword, two scrolls that may or may not be cursed, and an ox-cart with four oxen. Where do you even begin? The kind GM will play the merchant so as not to rip you off too badly, but really, if the NPC you're selling to offers you 230 silver nobles, what's your best response? No matter what your counter offer is, you risk being laughed at for demanding an exchange ridiculously in your own favour, laughed at for being such a dupe and not realising the true value of your goods, and then additionally not being sure exactly why it is that you're being laughed at.
That's when a little backup comes in handy.
Qui-Gon: Here's the money for the ion cannons. Deliver them to our ship.
Watto: You mean my ship. You said if I agreed to throw the race—
Qui-Gon: You didn't throw the race.
Watto: You told me not to race at all - after you gave me the ship.
Qui-Gon: We gambled for it. You lost the chance to throw the race. Ergo, the ship is ours.
Watto: Would you like to take it to the Hutts? I'm sure they can settle this. And I can collect the bounty on you while we're doing it.
Qui-Gon: Hmm. I've got a better deal. I give you the money for the ship, and we call it a day.
Watto: Why should I?
Qui-Gon: Laser sword.
Watto: <grumble> All right. Just give me the money and go.
Qui-Gon: See, they are good for Diplomacy.