Ah, I understand. Any NPC that will buy things from you will pay the same rate. No special rates from them. Just find the nearest one and sell to them.
The NPC vendors all have the same prices on all of their wares. (There was a time where some were different, but that was when specific vendors sold specific things, and you'd have to go to different zones to get items... that was a mess, and I think they missed an NPC when they changed prices, so they were cheaper)
As far as getting materials that are sold by NPCs, it is usually significantly cheaper to buy them from guild stores, provided you can find them (I find mine sell out really quickly, when I list them) (Trait stones being one exception)