1. The extra removable seats for an Audi A6 Avant 99-04 are available in this ad:

Audi A6 Avant 99-04 7 seat conversion removable seats for sale; original Audi version | in Crediton, Devon | Gumtree

2. A spare detachable tow hitch for the same car fitted with a Westfalia tow bar frame is here:

Towbar (removable) for Audi A6 Avant | in Crediton, Devon | Gumtree

In the interest of openness and honesty I should add that it is me selling them, but I am unable to post them on the 'sales' part of this forum as I have not posted enough times.