I am level 41, played with my character for 600 days yet my mercantile skill is still at 29. My speechcraft is also quite low but this could be my fault since i never used that skill very much. However Mercantile is one of the skills you use constantly( like athletics) since you buy and sell items throughout the game. I am quite frustrated because it seems that the only option i have to raise that skill is trainers (something i never liked to do). Any tips?

Are you bargaining or just trading at the opening price? Mercantile raises by selling for higher or buying for lower than the traders first price.

The skill is raised only if you bargain?

It goes up only if you bargain or bribe. This is controlled by the skill up value in the CS.