Like it seems like there are some NPCs with different prices, such as Enthir at the college, is it just because he is considered a black market dealer? Is there a hidden disposition rating? If the merchant has a quest and you do it, do you get better prices?

how many perks you invested in the speech tree, and how high their speech skill, for example, enther has a high speech skill, so his prices are really high.

My first playthrough, I noticed his prices and deals are nothing shy of a ripoff. I guess you pat for the convince. As far as I can tell, he is the only one that puts it to the player (so to speak). Everyone else should be on the level according to your speach skill.

Ahem. Speech Skill, Haggling Perks, and Price Enchantments.

Yeah, that's about it. No discounts that aren't perk-related. And oddly, once you have the Merchant perk, the merchants pay as much for stuff they don't sell as stuff they do. An alchemist will pay as much for a sword as a smith will.

Enthir will always sell at inflated prices until you finish the DB quest. Then his prices will fall to normal.