Chris,
I wouldn't buy them from anybody except a known supplier (i.e. B&H) or a trusted brick and mortar electronics supplier. Memory cards are some of the most highly counterfeited items in the world and online sellers (this includes the really big providers of goods of all types) are not good about keeping counterfeit items out of their system.
I run a lot of photos through mine but very rarely actually remove the card from the camera. I shoot most sports events with two 1DX II and one 1DX III body and the only time I pull the card is when I need to put one in the PC card reader to transfer a firmware update to the camera body. Otherwise, file transfer is done with the ethernet ports and all of these cameras are using the cards I bought when I bought the camera body. Each of my 1 series have both slots filled and I do rotate which slot is written to and I format in camera prior to an event if the card is down to less than roughly 50% left but that is it for my card maintenance. SD cards have a more rugged mechanical interface and I am less worried about flipping these cards in and out (which is what I do with my 5DS series bodies but they don't get nearly the heavy use of the 1 series).
I don't shoot video with my 1 series bodies so I have never come close to filling a card at an event. Cards don't last forever but you don't need to constantly rotate them and the reliability will be higher leaving them in place. They are expensive so I treat them well, it takes a lot of photography use before the card is going to have issues from write fatigue.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
