Yep , That sucks.

I remember i bought a heap of MOSC Gobots once (About 20 in a few lots) the seller just sent them in one of those large parcel post plastic bags with no protection.

I managed to get in touch with the seller, She turned out to be a little old lady that was just selling someones collection, (son or nephew or something).
She was a little upset, She chased up the post office for me and said they would have a look and assess the damage. I never bothered with it, the way they were sent i'm surprised they weren't completely destroyed.
The problem was, she just had no idea that the card back condition mattered to me.
Probably thought i was going to rip them open when i got them.

In your case, if you paid via Paypal you should be able to get the money back.. pain in the ass though,, sorry