Rule 1. Do not ever buy from npc, it can be made by players cheaper. And that goes for potions as well as armours and weapons (Dream of Mirror Gold) which on top of that also will be better. We can't think of more rules right now, but anyway try killing some low level mobs you can kill in 1-2 hits and keep doing so until you have earned a few thousands worth of drops.
Then you can collect some materials and sell them to players, you should be able to make 50% profit easily and 100% if you have some patience. One of the best places to do this is the well underground kill bats in huge masses and collect those red strings for 2 hours.
You will earn a few thousand. Also, you can try marketing yourself. Find items that are selling for low prices and then try selling (Dream of Mirror Online Gold) them yourself for a bit higher. Back in the day, you were allowed to open up 2 clients so you could have one resource collector and one main playing at the same time.
This was probably the most useful thing since you could earn money selling crafted or collected items while you were training, but apparently it was being abused. Now people just leave their clients on over night or when they are away just so they can earn a bit of money.