You can have all the legal rights in the world, but if you cant exercise them they are worthless
It depends what your parents signed
Its possible they signed to say it had arrived ok
In this case you dont have any rights to send it back at all
(the seller is responsibile for ensuring it arrives safely - if your parents said it did, the sellers responsibility has legally ended)
However - its quite possible that the box was ok but the contents were smashed internally during transit. In this case the seller might accept your word that this was the case and that when it was received it looked fine, it was only when opened did you become aware it was smashed.
If the didnt then legally you are entitled to send it back at no cost to you.
Problem is - the only way you could do this is pay the postage yourself and claim it back from the seller. If the seller dont pay it then you are stuffed. you would also be relying on the seller sending you a replacement
One option is to advise the seller whats happened and see what they say. It may well be that they will rely on your word and just supply a replacement without requiring the original sent back (whats the point of sending a smashed item back except to verify it WAS smashed?)
(take photos of object and offer to send via e-mail - as a buyer it was in your interest to have the object undamaged. Offering photos would show that the object WAS damaged and, as it would NOT have been in your interest to damage it yourself, then thats how it arrived.)
However the only thing you can do with certainty is raise a case with e-bay and get your money back (but I would still advise the seller first, cos I would not be happy if a case was raised without the opportunity to rectify the problem)