I'm presuming even though collected,  you followed eBay's rules and paid through eBay checkout?

Then,  there's nothing else you can do if the seller is asking,  for you to mark as collected.

The item was listed as a postage item so would not have a collection code a seller needs to scan to prove collection.  There's nothing the seller can do to mark as dispatched and their payout will be held for a minimum of 14 days.

@nicky460_1