If you're a new player or if you have no money to begin with, you can make money from Hidden Elf Forest. I've made two posts on it on the IJJI forum.
If you have some money, merchanting is almost always the fastest (the only exception I can think of is when the roger's abuse was first discovered). The more money other people are making, the more money you'll make, especially if there are very few extremely rich players.
You can sell polks (now Gcoins) since people always need skill resets, restoration stones, and regrade scrolls. You can try buying unfortified, ungraded items and fort/grade them. Well-graded items are hard to get, but they can sell high. If you do decide to sell Gcoins... *points to self*