What is the best course of action when you've ordered an item on ebay and it arrives so badly wrapped that pieces may have fallen out in transit?

I ordered a big lego set and there's a serious amount of damage to the box, enough to create a large tear. The lego box was not covered in any protective material (not even brown paper!) and the contents were wrapped in a black bin bag which has split at some point. If the box was held the wrong way up, pieces could have fallen out. What's the best course of action for me here?