Just wondering, I feel like I'm getting ripped off when I see something worth 2000 gold, then turns out it get's marked down over 50%

I'm pretty sure all NPCs buy stuff for the same amount of gold. The amount of gold you get for selling each item is dependent on your speech level and the perks you pick in the speech skill tree (which I severely recommend you NEVER go investing into).

At most, with 100 speech skill without any fortify speech items and perks, you'll get half the gold of what the item is worth.