You don't need to buy postage through eBay, buying postage at your local Post Office is just fine, that's what I do.
This is how to finalise that transaction:
You need to do this now, only on the Seller Hub
https://www.ebay.co.uk/sh/ovw
go to Orders >> Awaiting Postage / Print Postage label, and then click the box on the left of the item.
https://www.ebay.co.uk/sh/ord/?filter=status:AWAITING_SHIPMENT
From the Postage tab above, use the drop down menu there, if the option to mark as collected is not there, then select Mark as Paid and Mark as Dispatched.
Be aware too, today is eBay's maintenance day and many things go awry from time to time, especially early in the mornings here, where in eBay's HQ in CaliforniA , it's still early dark hours of the morning, and that's when updates are made, making things here in the UK not possible to view / manage from time to time.
So, if dispatched is missing now, then check a little later on, things should then be back to normal.
@fmj1976