I make profit at 3 ways.
1 - Hunting. Hunting for xp at places that gives you profit is the best investment you can make. When you level your character your damage, life, mana and capacity increases. You'll get access to better equipments equipment and new bosses or hunting grounds. The higher your level the higher is your profit.
2 - Bosses. This is a risky time investment and the drop chances are very low. I've been killing Scarlet for some time now with 0 lucky on expensive itens items for example. I gave up killing Oberon because of the low chances and because the time to reach to the boss is too high. When the boss killing requires too much time I think it's better to go hunting.
3 - Events. There are events where you can farm a lot of gold by selling the rewards. The orctobeer fest for example, i sold 3 plushies that i farmed at the first day for a huge profit. The cake event and dishes are also nice to farm as the life and mana foods are quite expensive. There are other examples such as First Dragon, Pearch Queen, demon's lullaby, etc.