This cheat also goes with the duplicate weapons cheat.

When you're near a merchant unload all weapons from your inventory but one, it should be something you can duplicate and is worth the most. I use the machete and sometimes Gabriel's sledgehammer.

Duplicate that weapon and then keep duplicating the weapons and storing them. When your inventory is full duplicate all the weapons in your inventory. You'll get hang of it after a lil while.

Keep duplicating that until there are hundreds just feet from the merchant. Then fill youe inventory and sell them to the merchant. Repeat this as much as you want.

As you level up weapons become more valuable, so just replace you old weapons with better, more valuable ones and you will not ever run out of money.