>>723174415
NPC merchants give you bad prices when your mercantile skill is low, and good prices if your skill is high, while Creeper's prices are static and don't change since he doesn't have skills or disposition.
It depends on your skill vs. theirs, but most merchants in the game have pretty low skill. You need to haggle to get the best prices, which also requires 80+ disposition to be good. Like if an NPC offers to buy something for 100 gold, you can instead sell it to 150 or so.