The vague part: Find something in your inventory you have 99 of that you don't think you'll need. Look that item's price up in the market wards and sell it at a price lower than the lowest price.

A few specifics:

Got a level 30 you want to work on and feel like doing a small bit of grinding? Kill Molting Mitelings in Nophica's Wells. Their Diremite Web sells for EASY money. Undercut the lowest selling price and put it on sale. That thread is a staple for their recipes.

Got a level 40-49 that you need to grind on? Gather up a group of your friends and go Raptor killing. They're nice and squishy mobs that still give decent exp even in 1.20. The new recipes use their skins - you can quickly get dozens of Raptor Skins that you can sell for easy, EASY money.

Tired of those two? Head to Nanawa Mines with a level 35+ character. Head towards the middle of Nanawa Mines - there should be a long vertical hallway with a lot of Bomb Embers that regenerate at a crazy fast rate. Start beating those up for exp. Their Bomb Ash is needed by level 25-30ish Blacksmiths for Steel Ingots. Compare against the market rate and sell it.

Players with a lot of money that are leveling their Leatherwork, Blacksmith, or Weaver as a secondary will be buying your stuff up. Some of these players want to level their crafts without putting work into gathering the items, so people like you who gets these anywaywhile they're leveling are perfect to buy off of.

I'm thinking a few people don't want to directly help you because if they specifically tell you what they sell, you may end up causing competition for them, which means less profit for them. It is business, after all. :P