This glitch / exploit lets you duplicate any melee weapon, anywhere and anytime. All you need is the "Melee Throw" skill. It works like this:
Step 1: Buy the Melee Throw Skill
Step 2: Equip your most valuable weapon (or buy one from a merchant). The more valuable your weapon, the more money you will make!
Step 3: Throw your weapon (press the right stick twice) and when it is in the air you open up your inventory immediately. In your inventory you will see that the weapon is still there. You must drop it from the inventory. So basically it will throw & drop the weapon at the same time and that's how the glitch works.
Step 4: Sell all your duplicates, but also keep one for yourself. You can keep duplicating it as often as you like. If you buy the two backpack upgrades you can carry even more weapons and farm much quicker!
Always keep a repaired version of your favorite weapon in the inventory. Use the duplicates to kill Zombies and when the weapon is about to break you can make a new duplicate. That way you will never run out of weapons!
If you want to farm for money you should stay in a quiet area near a safe zone as the noise may attract enemies.