eBay reference number for the account entry. May return no value for some
account transactions, such as an administrative action like placing the
user's credit card on file. You can uniquely identify
the entries that the GetAccount call returns within the AccountEntries tag.
Specifically, the following fields, returned within the AccountEntries tag,
can be used in combination as a unique key: AccountEntry.Date, AccountEntry.AccountDetailsEntryType,
AccountEntry.ItemID, and AccountEntry.RefNumber.