I believe that you need to seriously consider merchanting with
OSRS gold, simply buy something that is used heavily in PKing like food, potions, arrows. . .whatever and purchase 2-3% under middle and market a couple percent over. 3-4% per day may not seem like much but you can probably get it 3 times every day, meaning 9-12% gain in the event that you hit it right.
It adds up extremly quickly, simply avoid runes. Something good to look for is when the supply does not cover the demand for a few days and
Buy RS gold then when the supply outweighs the demand. . .In those states the price occillates (sp?) And if you dont make a fantastic profit one day you can easily come back later on. A source of 90% vs need on average is a fantastic bet generally as you can get enough of it to sell later when the price rises and falls.