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How can I link eBay transactions with PayPal transactions?
When you get the transaction details from the eBay APIs such as GetSellerTransactions / GetOrders, you will get back the ExternalTransaction which has the field ExternalTransactionID. This is the PayPal TransactionID for transactions that were paid using PayPal. You can use this field to sync the information with PayPal IPNs, APIs and the TRR (transaction details report). PayPal has also added the eBay TransactionID, which you can also retrieve from the eBay API's. If you use the GetTransactionDetails PayPal API, the matching fileds are L_EBAYITEMTXNID for PayPal and TransactionID for eBay.
If you use LMS SoldReport, BuyerPaymentTransactionNumber is the PayPal TransactionID.
In rare cases, the ExternalTransaction container goes missing in the eBay APIs because of post back issues. In these cases, you can use the combination fields such of eBay ItemID, Buyer UserID to determine which PayPal transaction it corresponds to.