WOTC around April Fool's wrote:
Page 71-72 – Diplomacy [Substitution]
Replace text with “Roll for initiative.”
Page 74 – Forgery [Substitution]
Replace text with “You actually trained in this skill? Well, roll for initiative.”
Page 74 – Gather Information [Substitution]
Replace text with “Roll for initiative.”
Page 81 – Sense Motive [Substitution]
Replace text with “Roll for initiative.”
Actually the main good point I picked up from the last few points is that Appraisal is an assessment, Diplomacy is for haggling. If you stink at either, you are going to get the short end of the stick (i.e., trade a cow for apparently magic beans out of either ignorance, or that's the best your silver tongue can manage even knowing the relative values).
Perhaps tie merchants and how they adjust buy / sell %, to diplomacy.
Perhaps give PCs a wonderful widget or wand which they can target items with, a secret appraise roll is made and the results reported to the PC. Once only per item, no way to click an item 500 times and try to average out a price. Something like a message sent to the PC telling the toolset value of an item, plus or minus some variable amount, depending on how good or poor the secret roll might have been.
How handy for mundane everyday items, don't know ("You estimate this longsword to be worth 28gp"), as people are probably OOCly / meta familiar with many normal objects' prices, like arms and armor. However if you find a gem, art object, a magic miscellaneous item, or something not easily valued at a glance, whole different story. DM can know darn well that a pretty jade comb is worth 40gp and a pot-bellied little goblin statue is worth 50gp, and a pearl worth 100gp, but PCs have no line of sight to that until they show each item to a merchant, usually. Yes people can memorize gem values (hint: diamond is always 1st place, or tied for 1st), but having a bunch of PCs eyeball an item in order to figure out worth isn't a bad thing.
Bringing it to a merchant though, maybe the buyback is based on the lesser of a diplomacy-roll influenced choice of (1) true market value, or (2) value PC believes item has. In order to help make that choice, any time a PC uses his "appraise widget thingy" on an item, apply an INT like "(PCNAME)_APPRAISED" for the merchant NPC to notice in a conversation tree. So a violin with a "true" value of 100gp, and an "appraised" value of 85gp, you make a diplomacy check haggle with the merchant to try to grind your way up or down from call it 50% of 85gp. I.e., you believe in that price, so you try to talk the merchant up based on that knowledge, as opposed to the 100gp figure. The situation where a violin was still "true" 100gp, appraised at 150gp, you're haggling your way up/down from 50% of 100gp rather than 150gp, as the high figure would be unsupportable to the professional merchant.
...
Maybe the above though is a pipe dream, nice brainstorm but implementation = not insignificant effort!
I think it was in talking to AL the other day about a potion shop in NWN1 Arabel (possibly one of Zelknolf's?) that I had a good memory of server messages during dialogue either praising or deriding your PCs knowledge or personality in talking about how much or little you were in a position to influence bargains. Something like "your knowledge about the products serves you well, but you are unable to sway her much on pricing", or "if you knew more about the products, you'd probably be able to better persuade her." Apologies to the author of the actual messages, I am paraphrasing, and also extrapolating what MAY be making those messages appear (i.e., the checks are surely in a script well hidden from me!)
