First, go to an item shop and store ALL of your money. Then, go to a town that has really cheap weapons for sale in the weapon shop. Make sure you have a weapon equipped that is worth more than any of the shop's items. Go to 'Trade' and find the cheapest item that is the same type of one of your equipped weapons, and start the trade, BUT!!! When it says 'Is this ok?' say NO. TA-DA! 999,999 gold instantly!