Two decisions, not one
Sellers often collapse selling on Amazon into a single question. It is two. The first is your fulfillment model: who stores your inventory and ships each order. The second is your seller model: what you sell and where the product comes from. They are independent: any seller model can run on either fulfillment model.
Each choice buys you something and costs you something, and the costs sit in different places.
Choosing a fulfillment model
Fulfillment is a choice between two options. Let Amazon handle storage and shipping, known as fulfilled by Amazon or FBA. Or do it yourself, known as fulfilled by the merchant. Letting Amazon fulfill buys reach: your offers become eligible for the fast-shipping badge many buyers filter for, which lifts conversion. It costs you fees on storage and each unit shipped, and hands day-to-day control to a third party.
Fulfilling orders yourself buys control. It also buys margin when your own storage and shipping cost less than the fees they replace. You set the packaging and hold the inventory. It costs you conversion, because self-fulfilled offers rarely carry the fast-shipping badge buyers trust. Every late shipment lands on your own record. Self-fulfillment can be acceptable for oversized, slow-moving, or highly variable products where storage fees would punish you. For most standard-sized private-label products, FBA usually wins on reach.
Choosing a seller model
Your seller model determines what you list and how much of the product you control.