Keeping a position in the Buy Box is an ongoing challenge. Third-party sellers can 'steal' your Buy Box position if they beat you in: Stock availability, Price, or Customer Satisfaction.
With higher performance metrics than your direct competitors, you can increase your price and maintain your share of the Buy Box. If your competitors have better metrics, you’ll need to lower your price to keep the same Buy Box share.
There needs to be inventory of your listed item. Without stock, the Buy Box will simply rotate to another seller.