ROI (Return on Investment) measures how much profit you make relative to your total investment.
Good eBay ROI benchmarks: <20% poor, 20–50% good, 50%+ excellent.
Example: Invest $25 total, sell for $58, net $24.91 profit = 99.6% ROI.
50%+ per item is excellent. Under 20% may not be worth the time and effort.