Imho master merchant is much better than grandmaster merchant, at master you can often buy stuff below their base value, GM makes it fixed always at the base value no matter how many points you invest in merchant skill.

You can even abuse it to "forge" some money - selling and buying expensive items at the black market in Thjorgard. With master merchant and 11 skill points in it I was able to get 3k profit per sell>buy operation from 21k gold worth artifact. The more points the cheaper you can buy it.