Q: How do I get a merchant to sell me an item at its best price?
For most merchants, you need indifferent faction and 104 charisma. This is to say that 1) at any given charisma, a merchant will give you the same price at indifferent, amiable, kind or warm faction (but worse at apprehensive), and 2) if your faction is at least indifferent, you will stop getting better prices beyond 104 charisma.
Now I said most merchants up there. The first statement is always true, but I've observed two exceptions to the second. First, the merchants in your guild in your hometown only require 93 charisma. Second, there are a handful of merchants who require MUCH higher than 104 charisma. These are usually the merchants near banks and at entrances to cities. Examples are: Tubal at the Qeynos gate, Boomba the Big at the WFP gate, Aimee at the NFP bank, Merchant U-something (the dufrenite merchant) outside the Kelethin bank. Don't buy or sell from them if you can avoid it. At 170+ charisma and indifferent-to-warm faction, I still can't get them to give me best prices.
Q: How can I tell if a merchant is buying or selling an item at its best price?
Check how much the merchant offers you for a peridot (Everyone carries a stack of peridots for their clerics and chanters right?). If you're offered 9.524pp for it, you've succeeded. You won't be able to buy for less or sell for more. If you're offered less than 9.524pp, you can do better with higher charisma. Now it doesn't have to be a peridot. Any item will do as long as you know its best price. I happen to know what the best price is for a peridot.
