Most picking mistakes are arithmetic mistakes. A reseller finds an item, feels it's "worth about $200," pays $120 for it, and discovers after fees and shipping that the $200 was an asking price, the real sold price was $150, and the profit was gone before the listing went up. The fix is one habit: decide your maximum buy price before you negotiate, by working backwards from the sold comp. Here is the formula and how to run it.
The formula
Max buy price
Start: the expected sale price — the median eBay sold comp, all-in (item + shipping), condition-matched, last 90 days.
Subtract: your selling costs — marketplace fees (eBay's final value fee runs around 13% for most categories; check the current schedule for yours), plus any shipping you'll absorb and packing costs.
Subtract: your target profit, expressed as a share of the sale price.
Result: the most you can pay for the item and still hit your number. Above it, you walk — no matter how nice the item is.
If you think in margin-goal terms — "I want at least 40% of the sale price left after the buy" — the shortcut version is: max buy = median sold comp × (1 − margin goal), then sanity-check it against fees. Both versions are the same idea: the comp decides, your goal disciplines, and the asking price is irrelevant until the end, when you compare it to a number you already trust.
Worked examples (illustrative numbers)
The items and prices below are invented to show the math clearly. The method is the point; run it with real comps for real items.
- A stand mixer. Sold comps, used and all-in, cluster at a $180 median. Fees at roughly 13% take about $23. You want $70 of profit in the deal. Max buy = $180 − $23 − $70 = $87. The seller asks $100: you can offer $80 with a clear conscience, and if they hold at $100, you walk — at $100 the deal pays $57 before your time, and your rule said $70.
- A video game lot. Median comp $60 all-in. Fees ≈ $8. Target profit $25. Max buy = $27. Small items punish sloppy math hardest: overpay by $10 here and you've given back a third of the deal.
- A vintage watch. Median comp $450 all-in. Fees ≈ $59. Target profit $150. Max buy = $241. The seller asks $300 and calls it "a steal at half of retail." Retail is not a comp. At $300 your profit is $91 on a $450 sale with real authentication and shipping risk — your formula already told you the answer.
Why the margin is a percentage, not a feeling
Set your goal as a share of the sale price — many pickers work in the 30–50% range for the buy-versus-sell spread before fees — because percentages scale. A flat "$20 profit is fine" rule overpays on cheap items (where $20 might be the whole price) and under-disciplines expensive ones (where $20 is a rounding error on the risk you're carrying). Your exact goal is your business decision; the formula's job is to make the decision before the driveway, when you're calm, instead of during the negotiation, when you're not. This is the same math FlipWire runs for members: every flip alert carries a margin goal the member sets (20/30/40/50%), and a max-buy figure is only shown when a condition-matched sold comp supports it.
The four ways the formula gets broken
- Comping wrong. The formula is only as good as its input. Asking prices, new-condition comps for used items, or one outlier sale will poison it — run the sold-listing method first, every time.
- Forgetting shipping. All-in means all-in, on the comp side and on your cost side. If you offer free shipping, that cost comes out of your side of the ledger.
- Moving the goal mid-negotiation. "It's basically $85, and I said $87, and it's really nice…" The formula's entire value is that it's decided in advance. Renegotiate with yourself at home, not with the seller in front of you.
- Paying for potential. "If I clean it / fix it / find the right buyer" is a different, riskier deal layered on top of the first one. Price the item as it sits; let any upside be a bonus, not the plan.
In the field
Nobody does long division in a driveway, and you don't have to: comp first (median, all-in), multiply by (1 − your goal), glance at fees, and you have a ceiling in under a minute. The full field workflow is How to Spot a Flip in 30 Seconds. What matters is that the number exists before the conversation starts — sellers can move you off an asking price, but they can't move you off your own arithmetic unless you let them.
Common questions
Should my margin goal be the same for everything? It can be, and simplicity helps discipline. Some pickers demand more margin on fragile, slow, or hard-to-ship items (more risk, more work) and accept less on fast, certain sellers. Adjust by category if you adjust at all — never by mood.
What if the seller's ask is already below my max buy? Buy it, politely, without haggling yourself into a worse relationship. The formula cuts both ways: it also tells you when to stop negotiating and shake hands.
Does the formula guarantee profit? No — nothing does. Comps describe the recent past, items break, markets move. The formula guarantees something narrower and more valuable: that every buy you make was sized so the deal works if the item sells the way its comps sold. That's the most any buying rule can honestly promise.
What about the small costs — gas, bags, tape, market fees on the buy side? Real, and they come out of the same profit. Most pickers fold them into the margin goal rather than tracking pennies per item: if your running costs are meaningful, demand a few more points of margin and let the formula absorb them. What you can't do is pretend they're zero while also wondering where the money went.