The formula
Profit on one sale is what you collected, minus everything that sale cost you:
Profit = revenue − platform fees − shipping you paid − cost of the item − supplies
Mileage is a cost too, but it is usually tracked per trip rather than per sale, so we keep it as a separate record below. Each term is easy to get slightly wrong, and small errors add up across hundreds of sales.
Each term, defined
- Revenue: the money that reaches you from the sale, before fees. That is the item price, plus any shipping the buyer paid to you. It is not the sales tax: marketplaces collect and send that on, so it never belongs to you.
- Platform fees: commission or selling fee, plus any per-order fee. Each platform charges on a different base, and some include shipping and tax in it. The Fee Ledger lists every base with its source.
- Payment processing: on Whatnot and Depop this is a separate line (2.9% + $0.30 and 3.3% + $0.45). On eBay, Poshmark and Mercari it is built into the main fee. Count it once, wherever it sits.
- Shipping collected versus paid: on Whatnot, Poshmark and Mercari the buyer’s shipping usually buys the platform’s label directly and never reaches you. On eBay and Depop you often collect it and buy your own label, so the two numbers can differ. In our example you collect $7.00 and pay $6.20, leaving $0.80.
- Cost of goods: what you paid for the item. When you buy a lot, split the lot price across its items (see below).
- Supplies: mailers, boxes, tape, labels, top loaders. A per-order average is fine; we use $0.85.
- Mileage: date, purpose and miles for sourcing runs and post office trips. Log the miles as you go.
Worked example: one $40 item on five platforms
You bought a lot of 10 items for $60.00 and sell one for $40.00. The buyer pays $7.00 shipping where the platform charges it, and where you buy the label it costs $6.20 (an assumed price). No sales tax. Fees are computed with the same engine as our fee comparison; the cost, label and supply figures are our assumptions.
| Platform | Collected by you | Platform fees | Label you buy | Cost of item | Supplies | Profit |
|---|---|---|---|---|---|---|
| Whatnot Buyer’s shipping buys Whatnot’s label | $40.00 | $4.86 | $0.00 | $6.00 | $0.85 | $28.29 |
| eBay You collect shipping and buy the label | $47.00 | $6.79 | $6.20 | $6.00 | $0.85 | $27.16 |
| Poshmark Buyer pays Posh’s label directly | $40.00 | $8.00 | $0.00 | $6.00 | $0.85 | $25.15 |
| Mercari Buyer’s shipping buys Mercari’s label | $40.00 | $4.70 | $0.00 | $6.00 | $0.85 | $28.45 |
| Depop You collect shipping and buy the label | $47.00 | $2.00 | $6.20 | $6.00 | $0.85 | $31.95 |
Two lines worth tracing (computed). On Whatnot, the buyer pays $47.00, but $7.00 goes to the label, so you collect $40.00. Fees are $3.20 commission plus $1.66 processing, and after the $6.00 item cost and $0.85 supplies you are left with $28.29. On eBay you collect $47.00, pay $6.79 in fees on that full amount, then $6.20 for the label, ending at $27.16.
Splitting a lot’s cost
The table uses an even split: $60.00 across 10 items is $6.00 each. That is simple, but it can flatter your best items. A second method splits the lot by expected sale value. If the $40.00 item is the standout and the other 9 are worth about $8.00 each, the expected total is $112.00, and this item’s share of the cost is $21.43. On Whatnot that lowers its profit from $28.29 to $12.86 (computed). Pick one method and use it every time; switching between them makes your numbers hard to compare month to month.
Common mistakes
- Treating a 1099-K total as profit. The form shows gross payments, before fees, refunds and what you paid for the items, and it can include shipping. See our 1099-K guide for what it contains.
- Forgetting shipping you cover. Free-shipping offers, shipping discounts on offers and Whatnot giveaway labels are real costs. A giveaway label belongs to the show it ran in, so subtract it from that show’s total.
- Not splitting lot cost. Putting the whole lot on the first sale makes that sale look like a loss and every later sale look free.
- Counting buyer-paid shipping as income without subtracting the label it paid for.
- Ignoring refunds and returns, including the return label if you paid it.
Keeping the records
Download each platform’s fee report or statement and keep it with your receipts, supply purchases and mileage log. This page is about record-keeping only. Whether something is deductible, and at what rate mileage counts, is a question for a tax professional.
Run your own numbers in the Whatnot, eBay, Poshmark, Mercari and Depop calculators; each one takes a cost and shows profit.
Fee rates as of Oct 2026, from each platform’s official help pages (checked October 4, 2026). Worked examples are estimates computed with our fee engine using assumed costs. Record-keeping information only, not tax advice. Flip Profit HQ is not affiliated with any marketplace.