In the situation where the buyer never pays for an item, the seller may receive final value fee (FVF) credit. The process for getting this credit is documented here.
You can use GetAccount to get the FVF that was credited back. The AccountEntries.AccountEntry.AccountDetailsEntryType field will specify the type of fee. The value of CreditFinalValue will indicate that the entry represents a credit issued by eBay for the Final Value Fee a result of an Unpaid Item dispute, under some circumstances.
The AccountEntries.AccountEntry.NetDetailAmount field will indicate the value of the fee. For credit the value will be negative.
Note that if you request a FVF Credit, the value of Transaction.FinalValueFee returned by GetSellerTransactions will not change for the item even if a credit is issued.