It is sad to hear of your experience.
I would like to add, however, that vaccinating a young dog will not protect it against CPV until it's immune system is able to build up an immunity to the vaccine. Generally a puppy with one vaccination that is challenged with CPV will succumb to it. The vaccine DOES reduce mortality to a degree, but it doesn't protect the puppy from CPV. And most puppies who contract CPV have a very poor prognosis.
Unless the petshop knowingly lost some puppies thru CPV at around the same time they sold your puppy (quite possible) there is little they could have done - yes, it is very possible the sold a "time bomb" to you.
All in all very sad.
what makes it even sadder is that, judging from the level of hygeine and quarantine measures (ROFL) at most petshops, the virus would have infected almost every dog that was placed in the same cage, used to same bowls or was handled after, a clinically sick puppy.
that is sad.