For Mercari, Yahoo! Auctions, Yahoo! Flea Market and Rakuma. Put in the sale price, what the item cost you and the shipping, and see what is left after the platform fee. Everything runs inside your browser; nothing you type is sent anywhere.
Profit (per item)
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Margin (of the sale price)
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Break-even sale price
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Time you can spend on it at ¥1,000/hour
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The payout fee is charged per transfer, so if you cash out several sales at once the share carried by one item is smaller than this (the page puts the whole fee on one item, which is the cautious way round). Fee rates change from time to time. The platform fee is worked out with fractions of a yen dropped. Check your marketplace's terms for how it really rounds.