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I listed my item using the PaymentMethod as IntegratedMerchantCreditCard but the GetItem response does not show that as the payment method.
Thats right. When a seller successfully uses AddItem/AddFixedPriceItem with IntegratedMerchantCreditCard, then for the resulting item, IntegratedMerchantCreditCard replaces specific, credit-card methods such as VisaMC, Discover, etc. In such a case, the listing displays (to potential buyers) the credit cards that the seller specified in the seller's preferences for their payment gateway account (in My eBay). Additionally, a buyer's credit-card payment is integrated into eBay checkout. Applicable only to the US site.
When the item sells, the transaction related calls such as GetItemTransactions, GetSellerTransactions will return