Your car is out of warranty, so you would have to make a goodwill claim for BMW to replace the bottle free of charge. I think they should come to the party with the cost of replacing the bottle, and by all means try that, but I doubt you will be successful. There are many plastic components used in the cooling system on BMWs, and like the expansion bottle on your car they all eventually give trouble. In my view it's poor engineering.

An older BMW I know of doesn't use any oil at all between services, but I'm told that it's quite common for the newer engine like yours to burn oil. It's generally held that a consumption of up to 1 litre per 1000km is normal. If you are concerned that yours might be too high do a check of the actual consumption. That way you know for sure how much it is burning.

If the damage were only slight, as you suggest, I would think it would be better to have it repaired before you advertise it. It would certainly make it more attractive to a potential purchaser.