Is there a way to shut down the horrible haggle system or do you know of a mod that does that?
What kind of game allows for worse prices if your barteing skill improves? I'm really fed up with this junk.

One that involves rolling a random dice opposed check. The same way you can fail savings throw that you made at a lower level when your saves were worse. RPGs are all about random dice rolls.

Generally speaking higher appraise equal better pricing, but if your appraise goes up 1 point, and you previously rolled a 10, and this time you roll a 1 for your opposed check, then you get worse pricing.

Overall it averages out, but for short intervals it may reverse a bit on the turn of the dice.

This seems like a minor thing to stress over.

Well, this is okay for p&p. I might even accept it if the dice were rolled every time you talk to the trader. But as it is, in a computer RPG it's just annoying.
Anyway I don't see how randomly different prices from the traders improve the role playing experience.

I agree with the post. That's a terrible, terrible system. You can wear an item that gives you +5 Appraise and still get worse bargain.

However, the check is only made when your Appraise skill increases. So, you can buff your Appraise before talking to a merchant (and load as many times as you want until you get a good roll) and later you will get the same prices even if your appraise skill is low.

EDIT: Source: http://nwn.wikia.com/wiki/Appraise

It's not very volatile. You are locked in until your appraise changes. Which means stable prices for quite a while typically, because why waste points on Appraise in the first place, so typically Appraise doesn't change much.

Plus if you are really that worried about about a bad roll, you can just use an intelligence potion, and get another roll.

It's trivia, it isn't like money is in short supply in much of NWN.

The whole system is annoying, and nothing will change my mind about that.
I agree that you don't waste points in appraise. That means if you get a bad start you're stuck with it for eternity. I don't mind bad prices for low skill, but I do hate the randomness in a matter that has nothing to do with the story. Selling loot and buying equipment is purely a matter of game mechanics and should be reliable.

Still my question stands. Does anybody know how to change it or if there is a mod that does it?

You could probably cheat your Int up for the reroll, then lower your Int to its normal value.

Alternatively, just save/load before first looking at a shop (or before each time the Appraise would be rolled for a shop).

Looks like you'd have to edit the script that opens each store (usually in a conversation). If you know how to code that's where you'd start (I think you'd change the OpenStoreWithAppraise calls to just OpenStore, could try that).

I probably could do that. But since I have never looked into NWN modding I don't know which files to edit and what tools I need. Seems I can't open the original campaign in the official toolset. Else I'd just have done it.

Can you post a screenshot of what you see when trying to open modules in the toolset? The toolset is what you'd use...

Okay I found it. Didn't see the switch "normal modules" or "campaign modules".

One option: You can download a tool like Leto to edit your save file and give yourself an Appraise skill of 99. This will cause every merchant to give you the best buy/sell price that he offers, which is -30 points from base for sale, and +30 points from base for buy. (The typical merchant in the Original Campaign has a 150/35 buy/sell base price.) Just know that in the Original Campaign, the game will throw an error upon transition to a different module, like from Chapter 3 to Chapter 4, so you have to set the Appraise back to 0 before the transition, then back to 99 afterward.

Inversely, you can set the Appraise value of some or all merchants to -99 using the NW Toolset. You can also adjust the base prices to something you think is fair. (Like if you want to ensure the merchant gives you the neutral price of 35, you can set the base sell price from 35 to 5, and then the merchant's Appraise skill to -99.)

Hm, I don't really like using savegame editors, but editing the merchants might be an idea. Although probably more work than it's worth.

If you're going so far as to open the toolset and saving a changed file...I can just tell you the exact 2 lines of code to change to literally fix your problem (removing the randomness from Appraise). Edit those two lines and then rebuild the module (or rather, recompile all the scripts). You can see the result in The Aielund Saga where I did this for Savant (the author of Aielund).