[mothered]

from your personal experience, has it ever happened that a seller (other than amazon or apple) opens a perfectly sealed product just to see what is inside?

As with all methods, the sealed box Is not 100% fail-safe.

You cannot ensure the return will not be opened.

in the termes there is what is written : "CONDITIONS OF RETURN: Unopened packaging, unused object, intact product, that is to say "NEW". Items that do not meet these criteria cannot be accepted and will be returned at the customer's expense."

If they want it at all cost unopen they wont open it to check if its inside right?

Put simply, It says that goods must be returned In their original state and will only be accepted as such.

Whether or not they're opened, remains to be seen.