A "Ping" is to alert someone else there is an article they may be interested in. HairOfTheDog maintains a list of those who are dog owners or just enjoy dogs. When I put her name in the "To:" box at the top of my post she was "pinged" to this response/thread. She (or soemone else on the list) may be able to answer your other question.
Ping to the Doggie List means that someone is keeping a list of FReepers who love dogs and like to read stories about them. Hair of the Dog keeps the list, I think. Just drop him a FReep Mail and have your name added and he will ping you every time a dog story comes to his attention. At the top you your screen it will say in Red "Comments", or something like that -- click on the red writing and it will take you to all your "pings".
I don't know anything about your pet store, but I'd report them to the police, or to the Humane Society if you have proof that they sold you a sick dog.