buy low sell high(you'll need to know every price of every item and this variates depending on the stats of the items ofcourse), been doing that for like 6-8 years and made tonns and tonns of RP's with barerly spending any real life cash.
Might think you'd need a starting amount of money for that but you can easily get your hands on some pluion/edwin or lower lvl weapon parts which you can try to sell/trade and buy back something better.....its time consuming and you need some strategy for it but its worth it.....at least it always has been for me.