From eBay help:

Proof of pickup

Proof of pickup is required to protect you against item not received cases.

For proof of pickup you can:

Use the eBay app to scan the QR code the buyer received through email

Use the eBay app to manually enter the 6-digit code the buyer received through email

To scan the QR code:

Go to Selling on the eBay App.

Go to the Sold tab.

Select the Arrange Pickup banner.

Tap the Scan to confirm pickup button.

If the buyer doesn't have their email with the codes, they can also access the code by going to their Purchase history in the eBay app.

Tip

When you scan or manually enter these codes, the item will immediately be marked as delivered but you'll still need to mark the item as paid if you collect payment at the time of delivery.

If you don't want to use the app, ask the buyer to sign a copy of the eBay order form at the time of pickup.

As to how to do it if you don't have the app- no idea!