Quick answer: Online arbitrage on Amazon is buying stock cheaply from other online stores and reselling it on Amazon, normally through FBA. The first-sale doctrine gives you the right to resell a genuine product you own, and Amazon’s rules only ask that the goods are authentic, that you are cleared to sell the brand, and that your name is on the paperwork as the seller of record. The low-inventory-level fee still starts at 28 days of supply, but it now catches bulky products too and is calculated for each FNSKU separately. The law covers the first; distributor invoices and gating checks cover the second. Here the focus is on the online version: what it is, whether it is allowed, what it costs after this year’s fee changes, and how to run it step by step.