Lets take an example
you buy a dbox you have to sign for it, now you sign for it and you realise that it package seems to light or maybe a bit to heavy, if you open it and it is empty there is nothing that you can do
now this is my solution, it sounds farfetch'd but get a video camera and record someone opening the package and record key points on the box like royal mail reciept. then is you do get it and it is empty or full of bricks say, you have physical proof that he scammed you, you can then inform paypal and you should get your money back
Hope This Helps In Someway To Someone
Package Suspecion 
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