You cannot replace the door skin on the A6. If its gone rotten then find a new door. Audi's are not the same as some VW's where you can unbolt the outer door skin. My wife did have a MK5 golf for a year where the door skin was easy removed (made it a lot easier when I had an issue on the lock) Audi's doors are all welded together so impossible to remove just the outer skin
I've just had a look on ebay as I've never seen a A6 C6 skin before listed before (but to be fair I've never searched for one), however the ads where the Ebay sellers call it a skin is in reality the entire door frame +/- parts they have removed to sell them separately (make more money).
For example
https://www.ebay.co.uk/itm/08-12-AU...749042?hash=item3b106e8572:g:hcAAAOSwetxbG42y
If you want to understand how the A6 door is fitted & all the parts are bolted on then best buy an hour on erwin
https://erwin.audi.com/erwin/showHome.do