Mercantile is basically just for purchases, or sales of items that he has acquired through quests.

Evaluation is just a vague, guesstimate for items. He can't tell exactly what it might be worth, but can get a vague guess as to the value. It would more be like a person who sells jewelry seeing a diamond ring and guessing at it's worth without doing a thorough evaluation. I could guess the ring is $8k, but without looking at the diamond it could be plus or minus 20% of that. I was thinking that it would be more like if he sees a weapon or other item in a quest he could guess at it's worth to whether he wants to take it or not.

Sixth-sense is a gut feeling, like anyone has, but it is a little more pronounced and typically acted upon. It is definitely more off-hand than anything. That doesn't mean that he knows when a situation is advantageous or a disadvantage every time. Example: he stabs an opponent and it would seem like he has an advantage due to the wound, but that person becomes stronger when they're injured - he wouldn't know that but he would have a good feeling about the situation. On the other hand, if he knows of a famous Althanas member from rumors he would feel far more at a disadvantage in a situation like that due to their rapport, whether that's the case or not.