It's not an eBay problem. Basically Royal mail added new letters to the tracking number starting YA instead of TQ, the post office system wasn't updated to accept these new code formats so arent scanning. It was meant to be sorted today but people have been told it's being updated tonight. Parcels can still be sent but the post office are unable to provide the relevant proof of postage receipt. It's not just affecting eBay but vinted and even if you buy postage online direct from royal mail