Amazon can be used by the sellers, but the platform is absolutely not for free. You will have to pay them Amazon seller fees for using their services. There are many types of fees that you are quite likely to incur, and here are some of them:
Selling plan / subscription fees
Referral fees
Fulfillment fees
Additional selling fees (Inventory fees, Rental book service fees, High-volume listing fees, Refund administration fees, etc.)