When there is no buy box, there is no buy box rotation.
Instead, the customer must go to the "See All Buying Options" page and select what seller to buy from. Usually, the seller will pick to buy from the lowest prices, new, FBA seller.
If there is no buy box, then technically all the new buying options are your competition. Keep in mind prime customers will most likely choose FBA sellers for the faster and free shipping so you will have a better chance of getting sales than FBM sellers. Additionally, since there is no buy box, if FBA sellers are priced the same there is no way to tell who will get sales and who will not. Most likely sales will go to the seller with the lowest price.
You can learn more about why there is no buy box here.
If there is no buy box, who are my competitors?