Its not possible to directly pull the buy box percentage from the MWS api.
The only way to do it through the API is to keep track of a buy box and record how much time a person has that buy box from the MWS API. This would require:
- Price Change Notification
- A way to record time of buybox
- A way to identify which listing is yours and which is your competitor and be able to map it 1 to 1
My point is its possible through the API but not very practical.