About the only way to make decent money on guns is to first buy them like a (real) pawn shop does.
You need a strong stomach for doing it, but you have to have a seller at their worst/lowest point in life,
then pay them about 10 cents on the dollar for their guns.
Example:
Back when a new S&W 586 sold for a little under $400. I watched a pawn broker 'negotiate' with a fella
selling one. The pawn broker offered $35., and was going to let the guy walk out if he insisted on $40.
I left and watched from across the street, a few minutes later- the guy came outside without it.