I fully understand your grief, and would be just as mad, but as you dont know the piggys history and in all likely hood he had been inbred, something I find nasty (I'm a prude) but any pigs I have had from breeders either rescued or given have died within 2 years most the first year, my only 2 left are clara and lily, out of 9 ( within 2 years) so it makes you think,
I know all breeders do not do this and am for breeding in circumstances like showing them and keeping them or giving them to friends so they have good homes or like in my case i know mine are not bred incorrectly if any of mine have any health problems then its simple I do not breed from that pig, I have not bred for about 2 years now I did try 2 females late last year but niether conceived but i only do it to top up my numbers instead of buying from shops, (which I do rarely)
have you tried telling the shop about simple guinea pig care ie not keeping rabbits in with pigs etc, I found a pet shop that kept their animals on sawdust I asked to see the manager and explained about sawdust being bad for them and he said he would make sure they are changed to paper and hay which he did do, he also explains about care to the customers and has a list of piggy lovers who can give them help ( me included) dont go back there and try to find another pig through the local paper, you will get to see their home and maybe their parents,
its very sad when a loved pet dies and you want to blame anyone else for it, but to be honest its the companys fault not staff as they have not had proper training, what about writing to their head office and explain that their companys got a bad name for itself and you are thinking of writing to a tv consumer programme etc about it,
I know its hard but be strong, the world is full of nasty individuals and we cannot hope to change them all , but knowing us lot on here are nice piggy lovers is proof there is some truly wonderful piggy and animal lovers around