Each product category has a new (and sometimes used) buy box. You have to have a professional seller account in order to be eligible for the buy box in any product category. Your order defect has to be less than 1% as well.
This is what the Buy Box looks like
How do I know if I have the buy box?
One way to officially know if you are eligible for the buy box is to check the buy box column in Seller Central. The buy box is moving around too often to know exactly the moment your product might have the buy box but most repricers (Demo video of Profit Reprice™) will be able to tell you when you had the buy box.