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PCLots of Monday morning QB's here.
Donald -- you're one of the good guys in this hobby, and I'm sorry to see you got taken here. Since the seller took PayPal, it doesn't seem likely that he tampered with the holder, but only the seller knows that for sure. In any event, the seller either did it, or knows who he got the tampered holder from. Definitely pursue the PayPal and credit card refund.
You might also contact SGC, not to recover on the guaranty (which wouldn't apply here), but to see if they are interested in helping determine who tampered with their holder. Perhaps the chain of ownership can be traced starting with the person who submitted the original/authentic Cobb card to SGC. SGC might not tell you who that person was, but they could follow-up directly. The way I see it, the first person up the chain of ownership that doesn't want to cooperate is the guilty party.
Don't write-off SGC's cooperation before asking them. I've seen cards with fake PSA flips on eBay before, and I've also seen people try to sell fake cards in tampered PSA slabs, in each case with full disclosure -- they were passing these of as "novelties". I emailed PSA on two occassions to alert them of this, and each time they killed the auction and sent me a thank you email. The top-tier grading companies have an interest in protecting their trade names and will at times police the misuse of their holders ... if PSA can be responsive I certainly think SGC will be too.
Good luck.