This post might not be allowed but there is no sticky for "For Sale" items in here as there is in the A3 8P section. So if i've done a wrong'un then sorry, please delete/lock or whatever you need to do :)
Anyway, my Dad sold his A6 C6 last month and had a few spares knocking around in the garage he has asked me to sell for him.
Here is a Brand New, Genuine LED Rear Passenger Side Light Cluster - Classifieds: Audi A6 (C6) LED Rear Light Cluster - New
I haven't got around to listing it yet but he also has a rear diffuser section that fits around the standard twin exhaust pipes (one at each side of the car). He upgraded the exhaust to a MTM or Milltek system I think so needed a different diffuser. I will list this soon but if you want to know the part number, drop me a PM.