The two richest merchants in the game are Creeper and The Talking Mudcrab.(There are a couple of people who have the same amount of money, but they don't accept most items)

Creeper
Creeper is located in Gorak Manor, in Caldera, and he has 5,000 gold in his inventory. The easiest way to get to him is to go to any Mages Guild teleporter and teleport to Caldera. Exit the Mages Guild and go south towards the second biggest building in town. It will be called Gorak Manor, and Creeper will be on the second floor.

The Talking Mudcrab
This Easter Egg is found North of Bal Fell, and east of Ald Sotha, on an island. He has 10,000 gold. If anyone has ever talked to M'aiq The Liar, he will also tell you of the Talking Mudcrab. Here is a link to a map, which shows the location of The Talking Mudcrab, made by DagothUrReturns:

Talking Mudcrab Location Map