We have been aware of an issue with postal tracking numbers in eBay. The ebay website sometimes displays the error "The tracking number you entered is already in use for another transaction and cannot be applied for this transaction. Please enter the correct tracking number for this order."
Having checked Google there are many forum threads with ebay customers reporting this error for valid tracking number. If you receive this error from ebay it is not a problem with the tracking number that we have provided for your letter. This is a ebay problem and customers should approach ebay directly.
Any letters that display this error can still be tracked as normal through the Royal Mail or USPS website, the only issue being that they cannot be entered into the ebay system.