Ebay sadly does not even provide you with a way to submit such evidence.
If investigative news meant anything, they'd be all over this. But most investigative news is gone, and there's so few investigative reporters and trustworthy news outlets left that its hard to hold ebay's feet to the fire.
Even if you had a film of the box being delivered to the post office you could have a corrupt post office friend or a post office worker that is scamming things on their end by opening packages and replacing the contents.
There is no 100% way to prove you sent what you say you have.
This is the same (sort of) reason that mailing yourself something is not proof of anything, as you can mail yourself a bunch of empty, unsealed, envelopes, then seal them with whatever inside them at some time in the future.