eBay UK said a glitch with Royal Mail tracking prevented some users from being able to track their packages last week:
We’re aware that some customers experienced issues with Royal Mail Track & Trace last week on eBay.co.uk. Some valid tracking number searches may have returned an error message: “Sorry, we don’t recognise that reference number.”
This issue was limited to a small number of queries. Royal Mail have confirmed that they’ve fixed this issue, and all valid searches should now return up-to-date tracking, signature and delivery confirmation information.
We regret any inconvenience this might have caused.
Source: eBay UK Announcement Board