Work backwards from the sale
Most bad buys happen because the price tag looks cheap next to what the item sells for, while fees and shipping are ignored. Working backwards fixes that. Start with a realistic sale price, take off fees and shipping to get what you would actually receive, then take off the profit you want. What remains is your limit, and anything under it is a buy.
Fees depend on the sale, not on what you paid, so the math is exact once you pick a marketplace. If the answer is zero, the item cannot reach your target at any price on that marketplace, and it is worth trying another before you put it back.
A worked example
You find something you expect to sell for $50.00 on eBay, you want $20.00 of profit, and shipping will cost you $8.00. After fees and shipping you would receive $34.32. Take off your $20.00 and the most you should pay is $14.32.
Using it while sourcing
Be honest about the sale price. Use what similar items have actually sold for, not what they are listed at, since asking prices run high. Sold listings are the number to trust. It also pays to set your target profit before you walk in, so an exciting find does not talk you into a thin margin.
Think about how long an item will sit. A higher target makes sense on something that may take months, because that money is tied up the whole time. A lower target can work on an item that sells in a day. It is also fair to raise your target for anything fragile, heavy or awkward to ship, since those take more of your time and carry more risk.
The page works on a phone and needs no login, so you can keep it open in the aisle.
Common questions
What does max buy price mean?
It is the most you can pay for an item and still hit your target profit after marketplace fees and shipping.
What target profit should I use?
Use the smallest amount that makes the item worth your time to photograph, list, store and ship. Many sellers also set a higher target on anything slow to sell.
Does this include sales tax I pay when buying?
No. If you pay sales tax when sourcing, keep your limit a little under the answer, unless you buy with a resale certificate.