You'd need to also record yourself putting a real phone IN the box when you're shipping it, but it seems that even if you did, your evidence would just be ignored by ebay.
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.