Lower level weapons eg fairy bows and Stonevale swords are highest at castle weapon seller near bank. There may be other Traders that equal but none are higher that I have found.
Skill merchants in castle for regular drops like eye irises or claws from Carro etc seem highest.
For bought Lux the wraith at crossing Ley in OW seems good but I always check a few because there have been exceptions where the wraith was quite low. But you would normally sell tese to players if you could I suppose.
I think there are (at least) two traders that sell high for each class of object.
Just my opinion.
Landoril 221 Ranger
Denoril 170 Rogue
Hateril. Hat holder
Pageril...well you get the idea
Epona (The little server that could)