The most "expensive" items I know that we can buy from NPC are from:
1. Black Bert, you can find him at the second floor of the Docks of Thais, here. And this are the 2 more expesive items he sell.
|Blood Crystal ||50000|
All the items you can buy from this NPC are part of differents quests, you can buy them if you lost the item needed in the quest you are doing at these moment.
But also some of the items he sells are also used for players to decorate houses, the most commons I have seen are: Silver Nuggets, Gold Nuggets, Ghost Charm, Crimson Nightshade Blossoms, Bloodkiss Flower.
Keep in mind that to trader with this NPC you have to complete the quest: The Thieves Guild Quest.