Selling the same last unit on two channels at once: one pair of shoes in the warehouse, an order from one marketplace in the evening and another an hour later. One customer has to be cancelled. The cost is not the product but the cancellation rate, which marketplaces track as a seller performance metric, and the review that tends to follow. Thresholds and consequences differ by marketplace and change over time, so check the current seller agreement. The cause is always the same: stock reaching the other channels too late.
See it before we explain it.
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