"Have you gotten a weapon from the Supply Crate MYST-N that just didn't tickle your fancy? Well don't let it go to waste! For the month of November, sell your NX Rare weapons back to us for GP!
Once the rare weapon is in your inventory, you'll find the "Sell Back" button among your options for each rare weapon. A click of a button, and you can trade your unwanted NX rare weapons for cold hard GP!"