Which merchant(s) will offer me the best prices for all of my stuff?

After testing on a representative sample of items (magical and non-magical weapon, armor, trap, potion and lockpicks) on several merchants (including but not limited to Defiance Bay marketplace, Gilded Vale blacksmith and inkeeper, stronghold merchants and one Vithrack)...

It is not dependant on reputation in the area (tested with manually changing Gilded Vale rep with cheats), chapter you're in (tested on transition between 2 and 3), type of merchandise they sell, the location they're in, whether you completed a quest for them or not. The sell worth of every item is fixed and you will get the same price regardless of the merchant you choose.

Yes Heodan pays a lot more than other merchants... but you can hardly sell him any loot. I think the idea is to change starting gear to what you want, he buys and sells at same price. There should be info about it though cause most of us don't care to buy stuff on start cause of low money and merchants usual margin.