Profit is the money left after the selling costs you include. ROI and margin express that result as a percentage, but divide by different amounts.
Work through one hypothetical sale
Suppose an item sells for $40. You paid $10, spend $5 shipping and $0.75 packaging, and owe a $4 seller fee. Profit is $40 minus $19.75, or $20.25.
In the public tool, cash invested is purchase cost plus seller shipping, packaging and other entered costs: $15.75 here. Seller fees are deducted from proceeds rather than included in that investment denominator. ROI is $20.25 divided by $15.75, about 128.6%.
Margin divides the same profit by revenue: $20.25 divided by $40, about 50.6%. The two percentages describe the same sale; neither is an additional amount of money.
Compare records with consistent definitions
A spreadsheet that includes fees in its investment denominator will show a different ROI. That is a definition difference, not necessarily a calculation error. State the denominator before comparing results from different systems.
Buyer-paid shipping adds revenue and may add selling fees. Marketplace-collected tax is not your revenue. The shared method explains each input.
Handle zero and missing data
A free item can still require shipping and supplies, making investment positive. If all investment is zero, ROI is undefined. Do not replace a missing purchase cost with zero and infer an exceptional return.
A loss should remain negative. A short selling history or incomplete costs cannot establish your typical business ROI.
Choose the comparison that answers your question
Use a dollar target when you want a specific amount left after costs. Use ROI when comparing profit with invested cash. Also consider time, uncertainty and how long stock sits; the calculator does not automatically model these.
Try the ROI tool and record the actual outcome when the item sells.