still doesn't make much difference. C units are not in high demand. You still didn't tell me what model this is for; have a part#? If it fits the A8 (4D0 unit) then you might be able to sell it. Good luck!
sounds like this is just the CD drive unit from the trunk....and not the Nav+ version. In my opinion, you have a very limited market for selling this. I think I recall seeing a used one sell on ebay for $200. Try your luck there and see if you can get a better price.
FYI - the people that I know who have had a C-Navi drive unit fail, have opted to spend the extra $$ to upgrade to the RNS-E....than spend $400+ for a new part from the dealer.