If the buyer checked out using "Local Pickup" they should have brought the eBay email with the QR code/6-digit code to scan or enter using the app. That automatically marks the order as "delivered".
If they didn't checkout using Local Pickup, or you weren't given the codes, use the dropdown to the left of the item in seller hub and mark it as "Shipped" or "Picked up", whatever the options are.
Read "Proof of pickup" here:
