FeeTrail is a free, independent marketplace fee and profit calculator for online sellers. It helps sellers estimate platform fees, payout, profit, margin, and ROI before listing a product.

Unlike calculators that return only a final number, FeeTrail shows each deduction, the amount used as the fee basis, the applied rule version, effective date, calculation assumptions, and links to official sources.

FeeTrail supports marketplace scenarios including Amazon, eBay, Etsy, and PayPal across US and UK markets. It requires no account or marketplace connection, and calculations run directly in the browser.