KBB also does not account for your CPO. Dealers in actuality jack prices way up if a car is CPO'd, so that's extra for your worth. You will still be in the KBB ball-part if you want to sell, however. Everyone seems to use it as a standard. The one thing I'd recommend is looking at a BMW dealership near you and seeing what prices they're charging for similarly-optioned/mileaged cars as yours, and that'll be your best estimate.
'11 6AT 335i xDrive ///M BMW PPK Stage 1