The server fails to check the quantity of the items that are going to be sell. Values <= 0 are accepted as 1.
Go here https://sandbox.reverb.com/fr/item/139897-fender-2-strap-leather-test-2018-leather
Intercept the response after clicking "Add to cart" and put "quantity: 0"
Proceed to checkout
I used one of the fake credit cards you provide us.
Items are sold gratis