If you search the for sale section you’ll find a couple of listings, such as this one: ND EXHAUST BACK BOX - SUPERB CONDITION - #2
They come up occasionally, but not sure if they actually sell or sell for the money they list for. I assume you are selling following an upgrade? The amount of miles on the exhaust system probably makes a difference, too.
I just kept my old system after putting a BBR one on in case it ever needs to go back to stock (or a new owner wants to do that).