If you bought before your account was fully set up or while logged out you would be classed as a guest buyer.   Here's more info:

https://www.ebay.co.uk/help/buying/returns-refunds/returning-item/returning-items-bought-guest?id=40...

Probably because you were not logged and bought as a guest, if so the only details you will have are in the email from ebay confirming purchase.

https://www.ebay.co.uk/help/sr?query=buy+as+guest&origin=5&aiSummary=true