Ok, so you want to make money, but you want to make it over time, there really is no such thing as, quick money. It usually made over time, and with a lot of good thinking.
So, you start off with some GP, go make some, say you have about 50k. You go out and buy say, a rune scimitar for about 27,000 GP. you get board of this item, you want to go get a dragon scimitar. You bought that rune scimitar for 27,000 GP, never sell for less that what you bought if you wanna make a profit. You bought it for 27k, go sell it for 28k, push 29k or 30k if you want to, but if you want a fast profit sell it for 1k more. When you get random drops from npc's, you can basically sell those for anything, but if you wanna make a decent profit again, sell for about what you would buy it for and if more. Doing this would help you make a decent profit over time, and you would make millions if you do this every time you go to sell a item.
Good luck and good selling