To win the Buy Box, first make sure that both you and your listed items are eligible.
Becoming eligible to win the Buy Box doesn't guarantee that you will win it; that said, there are a few things you can do to increase your chances.
There are a few ways to check and make sure you're pricing your items competitively.
Sellers who offer faster shipping options are more likely to win the Buy Box. You can look up your available options in Shipping Settings.
You can't win the Buy Box if you're out of stock. Keep your inventory updated and plan accordingly for things that sell quickly.
The Amazon Selling Coach can show you low and out-of-stock inventory alerts, letting you know, in real time, when it's time to stock up.
Listings that do not win the Buy Box are eligible for placement in the More Buying Choices box (but note that Amazon does not guarantee placement in either of these locations).