What reason did they give for asking for a return? Was something wrong with it?

At this stage you should have accepted the return although you would have been responsible for collecting it from the buyer.

If you did not respond to the case, Ebay would have closed the case and refunded them.

Have you contacted the buyer to say that you would like the scooter back?

You have only been scammed if the buyer has said that they did not collect the item when they did. But now you know that you have to enter a code for proof of dispatch.