Mine for cobalt ore. That sells really fast. Also, make steel ingots with iron ore and bomb ash. If you don't feel like mining, then look at different items and see what sells, then farm for it. On my server, Ahriman Wings sell pretty fast for a good price. But yeah, it really depends on what sells and for how much. Make sure to pop into the Atomos events whenever you can to see if people are fighting it so you can try to loot some over-aspect crystals, which sell pretty well.