Tag Match

Template:Verify Most NPC vendors buy items back at 80% of the item "value". A few select vendors will pay a full 100% of the item value. Portalarium calls this a "tag match". Any particular NPC may offer a tag match price for more than one item category.

To identify such vendors, simply see what a vendor will pay you for a given item, then see what a completely unrelated vendor will pay. It will help to have a single un-stacked item in your inventory, or you can control-drag a single item to the exchange pane. Items with a value of less than 5g may be inaccurate due to rounding.

Note that what any NPC will pay for an item with a durability stat is affected by the durability. This is mostly important for weapons and armor. A weapon at less than 100% of brand new durability will sell for a lower price, but is still subject to a tag match mark-up.

Alchemist

These vendors will offer 5g each for gold and silver ores, 200g for gems, and full value for potions.
Etceter: William Warren (79, 24, 38)
Kingsport: Alicia the Alchemist (90, 26, -3)
Kingsport: Julia the Alchemist (77, 25, 0)
Owl's Head: William (95, 72, 45)

Blacksmith

These vendors will offer 5g each for iron and copper ores, and 50g each for nickel, tin, and tungsten ores.
Etceter: Aaron Denbo (100, 24, 34)
Etceter: Ezb Krohon (66, 24, -30)
Owl's Head: Blacksmith (98, 87, 238)

Weapons and Armor

These vendors will offer a higher price for weapons and armor, up to full value for like-new items.
Aerie: Oakie Lawrence (-40, 35, 219)
Central Brittany: Combat Merchant (360, 32, 395)
Etceter: Aaron Denbo (100, 24, 34)
Owl's Head: Theo (93, 87, 226)