What is ROI in Amazon arbitrage?

Profit as a percentage of what you spent - the arbitrage decision number.

ROI - return on investment - is profit divided by what you paid, expressed as a percentage. Buy at $10, net $3 after all fees: that is 30% ROI. Arbitrage sellers think in ROI rather than margin because capital is the constraint - ROI tells you what each dollar you spend comes back as.

Common working floors in online arbitrage are around 30-40% ROI and a few dollars minimum profit per unit, leaving room for price dips and surprises. The right floor is yours to set; the point is to set one and check every buy against it rather than against enthusiasm.