on high rate. go mvp and sell mvp cards, they got high drops and such so you can get rich quick. On low rate, the best way is to buy then sell, and gain the profit, but you have to have suitable amount of money to begin with.
On a low raten sometimes, when i need money, i go hunt grand pecos. They drop mastela fruit at a high % and also drops gold. Another thing that i do is to cook lvl 3 food or above and vend them. They make nice profit if ur not lazy

but you also need chef hat and cooking set for these.
to be honest, what i do is keep an open eye on market, buy cheap sell at a reasonable price.