'I sold an item on 5th March, tracked and delivered 9th March.

E bay give this customer until 17th April to return this item.'

Perhaps that 'extra' time includes the time allowed for the buyer to post the item? If a buyer leaves it until the end of the 30 days to start the return, then has 10 business days to get it posted, it can be well over six weeks before the item finally gets back to the seller... by which time, other would-be buyers will very likely have lost interest/purchased elsewhere. Ah, the joys of eBay selling... 😬