Introduction
Skills that can help you in earning money for non-members on Runescape are Mining, Fishing/Cooking, Woodcutting, Smithing, Crafting.
Mining
A player who can go into the mining guild earns money fast. Sell Iron ores for 50 to 100 each, Coal for 150 each, Silver for 200 each, Gold for 150 to 300 each, Mithril ores for 250 to 300 each, Adamantine ores for 900 to 1k each and Runite ores for at least 10k each.
Smithing
When you've got a decent amount of money, you can buy ores/bars. Smith them to get your level up. It takes a long time, but you earn a lot after you can smith rune items.
Mining/Smithing
Recommended for new players. If you are new, mine copper and tin. Smith them into bronze bars in a furnace. Make bronze items out of them and sell them to stores. A good place to do this would be the South-east Varrock Mine/Al-Kharid Furnace/Varrock Anvils and Stores. For other players who do not want to spend their money buying ores and bars, this is an option for them. Just mine the ores you need and smith them into items, after which you can sell them..
Fishing/Cooking
Train fishing up to level 40 so that you can fish lobsters. Before that, fishing doesn't really earn much money for you. Fish lobsters and sell them. Cooked lobsters sell for more and are easier to sell so I'll recommend cooking them. You need a cooking level of 40 to cook lobsters. Sell cooked lobsters for 150 to 200 each. Swordfish is also a way of earning money, but less people buy swordfish. You need a fishing level of 50 to fish them and a cooking level of 45 to cook them. Sell cooked swordfish for 250 to 350 each.
Woodcutting
A skill that earns a lot of money, but you need a lot of patience. Keep training your woodcutting level by cutting trees until you can cut oaks. Then, keep cutting oaks. Even though willow trees give more exp, I've noticed that cutting oaks give better overall exp. Cutting oaks give more exp per hour because you miss less often, as compared to cutting willows. Once you've got woodcutting level 60, go cut yews. I would strongly recommend you use a rune axe before cutting yews, because it takes a long time. Sell yew logs for 250 to 350 each.
Crafting
Not recommended for non-members but it is still a way to earn money. Craft gold bars into necklaces/amulets/rings and sell them to the general stores. The only good way of earning money from crafting is by making diamond amulets (requires crafting level 70), enchanting them into amulets of power (requires magic level 57) and selling them for about 10k each. A good way of leveling up crafting is by making holy symbols, as they give 50 exp each.
Buying and Selling
Go to Varrock world 1 for some bargains. Buy items at low prices and sell them for higher prices. For example, I bought 5 rune 2 hand swords for 40k each and sold them for 45k-74k each. However, you need to have a certain amount of money or else you cannot use this method.
Fighting
Fight monsters and earn money from their drops. If you are a member, it is highly recommended that you range fire giants, as to the fact that they have lots of valuable drops such as rune arrows, rune scimitars, half keys, dragon left half shields and dragon mediums. Another method of earning money from monster drops in members is to kill monsters such as Hell Hounds for clues.
Another good way to make money is to fight Green Dragons in the Wilderness. This is only available to Members. Two common places used are the Wilderness North of Falador or the Wilderness Northeast of Varrock. Keep the Dragonhide and dragon bones and not anything else (unless it is worth over 2,000gp). The bones and Dragonhide are worth a lot and are always in great demand. Additionally, you may recieve a clue scroll which (as mentioned above) can also get you a lot of money.
Fletching
After woodcutting your own yews or magic logs, make them into longbows. After that go and get flax and use it on a spindle wheel to make bow strings, or you can just buy them for 100gp each. Use the strings with all of your bows. After this go mine rune essence or buy it for 20gp each. Go to the Nature rune alter after getting 44 RuneCrafting and make enough nature runes to equal the number of bows you have. After you have that, whip out your fire battle staff and high alch all of your bows. After that you should have about 400k total for every 1k yew longs and about 1.2 mil for 1k mage longs.
Common Scams to Watch Out For:
1.) The mistaken amount - Many times when you are doing a trade people will try to rip you off. One of the easiest ways is to make you mistake the amount or item. Sometimes, people will say buying something for 45k, when you go to trade them they put up 45k, then they accept, when it comes up to the "are you sure window" they retrade and quickly put only 4.5k up and accept quickly, hoping you wont notice. This trick also is common between Rune and Mithril items. So make sure to always recheck the trade!
2.) The two people scam - The best way to tell this scam is to show an example of it in action. First, two people devise a plan as follows. One person says they are doing a quest and indeed of a item but is too lazy to get it and will pay extremely good for it ( lets say 20k for a black bead ). They will announce this at a crowded area. Now a little down the road their buddy is waiting and types "selling black bead 5k". A player will see this and think "wow i can make an easy 15k" so they buy the beed from the person for 5k and go back to the friend and try to sell it for 20k, however the person never buys it, thus the two scammers make 5k off an item worth 5gp.