Think about it.I can't follow your reasoning: Isn't that exactly what would happen? The dealer would reduce the amount of a trade in value on the basis of condition? Or am I missing something? I've never seen a used bike in a BMW dealer with damage and a sign saying, 'Sold as seen'.
Used condition, in this case a 4 year old bike, normal wear, inc corrosion as this seems to be a regular issue on BMW’s. Trade in price and subsequent resale price reflect the wear & if present corrosion.
If the dealer had to replace all the parts that showed wear &/or corrosion before it could be sold, then either he is going to make a huge loss or have to sell the bike for more than a new one. The post above by MotoRevive explains it better than I did.
I doubt there’s another in the UK like this, it’s done a massive 650 miles since it was registered in 2023, ceramic coated to protect that lovely black engine paint and full ppf to protect that triple black paintwork. 
