Go into any shop that will allow you to sell items, select "Sell", and then press (and hold down) the "A" button, once it displays your inventory, without letting go of the "A" button, press the "B" button. A dialogue box will display asking if you would like to sell your Attack Knife. Select "Yes", and you'll be paid for the Attack Knife, regardless of whether or not you had an Attack Knife in your inventory (if you do have an Attack Knife in your inventory, you won't lose it). This can be done an infinite number of times to accrue large amounts of money.
Repeating the same process, but substituting "Sell" with "Buy", one can purchase items one should not be able to purchase from that shop (such as a Hiko-Bari from the DRUGS shop in the Kichijouji arcade at the beginning of the game).