It happens to many, that is, items left in full view of others, can and often do get stolen.
I'm afraid nothing eBay can do to enforce a safe spot for items to be left, that is entirely up to the courier / postie delivering to you.
Sellers pay extra for a tracked service so once that shows delivery to the delivery address on the order, the seller and eBay cannot be liable any further.
When I use a courier, I always receive a feedback to complete after the item has been delivered, and the courier's name shows asking how ' well they did'.
Do you get those, then you can politely inform the courier company that your item delivered by Mr / Ms ***** was not left in your designated safe place, and was subsequently stolen.
That's all you can do inform the couriers, and your postie where items Must be left.
@arrrrtex