Re: "a camera that has multiple card slots"
My Canon 5D Mark 3 has CF and SD card slots. A dozen of those might cost you a few quid though and that money would be better spend getting memory from a reputable seller instead. It's not the only Canon to offer this and other manufacturers offer it too. As for operating in RAID - I suppose it's RAID 1 since the card slots operate in parallel. I doubt you'll find RAID 5 in a camera unless you have it routed to a nearby NAS box via Wifi...
If a camera of that calibre is outside your budget, then test the card before use. Reading your original comment it sounds like you added media over several days to the same card before realising it had failed. Surely you would have checked the stuff you'd captured at least once a day if only a quick browse? At least then you'd have spotted the failure sooner. Also the camera ought to have detected a problem with the card. One of my CF cards failed once, taking with it 5GB of RAW files, but the camera at least told me there was a problem and I was able to swap cards.
Fulfilled by Amazon means nothing regarding how reliable/genuine the item is. They're acting only as the warehouse/logistics agent - the Marketplace seller is responsible for the product itself. Fakes may slip through - but this is less likely with Amazon supplied items and in that case you'd at least have comeback against Amazon themselves.