When you are somewhere you can sell something to someone go to the person, now look down at the floor.

Hold down LT, then press RT and Y at the same time.

You should drop the weapon you have and another one will drop, but you have to keep holding the buttons after for the second weapon to fall.

Then you pick up one of them, sell it and then pick up the other one on the floor.

Repeat this as desired to get easy money.