I have noticed that some merchants seem to pay you more for goods you sell to them than others. So far, the dwarf in the Underdark seems to pay the most (after you threaten him into cutting his margins). Where possible, I try and save up my valuable items to sell to him.

Has anyone ever worked out which merchants give the best deals (and which ones to rip you off)?

I think maybe Nym in the Wandering Village seems to be the stingiest.

I once had many high value items to sell. I was in Kuldahar. There are more merchants. And I made a excel sheet to sell to the one with the best price.

It was different with every item. I could see some patterns: once of them was a magician (or wanted to be) he had best prices for stuff that he could use. But not all of that stuff. Another one had best prices for arrows and other ammunition. But some types of ammunition had better price by other merchant.

I would tell - you cannot tell who had the best prices. You can tell who has the best price for one item.