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Why do I sometimes see a purchase (Transaction) appear in an Order when that purchase (Transaction) has already been reported by GetSellerTransactions or GetItemTransactions as Completed for eBay Checkout?
Transactions that have completed eBay Checkout can appear "again" in an Order.
The following describes the case when this can happen:
If a single item purchase (single transaction) or a combined item purchase (order) is made, but the transaction or order involved has not been marked as Paid, then what can happen if the same Buyer buys another item from the same seller and goes through eBay checkout, is that the ReviewYourPurchases screen will include all of the other purchases (transactions) from this seller that have not been marked as Paid (even though they have completed Checkout). Again, the key point here is that eBay Checkout will by default include all purchases from a seller, who allows combined payment, that have not been marked as Paid, even if the Buyer has completed Checkout flows for the other purchases.
This is generally not a concern when the Buyer completes the eBay checkout flow through PayPal since the transaction is marked as Paid by PayPal fairly quickly. This can be a concern when the Buyer will pay "asynchronously" such as via Money Order or some other method where the seller has to wait for payment to be cleared before marking the transaction or order as paid.