Well you could go to Toy Factory to farm cakes and make a merchant to sell them at 3k each.
You could go to YGGdrasil and steal blue herbs from shining / blue plants, go to the potion maker in Alberta and get him to make one blue potion for every 2 herbs you have, then NPC them on a merchant with the Overcharge skill for 3.1k each. [This was stolen from someone's zeny making guide, not my idea].
Or as someone said above, Amatsu dungon by farming star crumbs and NPCing them. [Another good reason to have a merchant with the Overcharge skill].