Just want to add that while the other websites will make a profit from your sale, they had to buy them from Theory11 in the first place. I don't think its any question of loyalty, you'll be benefiting Theory11 almost identically, and also another magic business. The only problem would be if they other seller was claiming it is their own deck, but that would be idiotic and you know Theory11 produced the deck anyway.
A little technicality here; Theory11 didnt design the deck, Simon Frouws did, by hand, in South Africa apparently. Theory11 didn't print it either, the USPCC did. What Theory11 did was produce it, on a basic level they financed an organised all the steps required to make a deck exist and to sell it too. Not to say that it isn't "Their" deck, a design that intricate requires a lot of work, and I'd only be guessing here but I suppose that whoever it is on the Theory11 team, probably a combination of all of them had a vision for what they wanted this deck to be, its up to them to communicate this vision to the designer who has the ability to physically produce the artwork, perhaps with a little of his own input.
Then they are responsible for letting the USPCC know what their design is, what stock they want it on, to use the FSC approved sustainable paper, and the cancer curing ink, then they have to market it, someone produces the trailer, designs the product page, adds the button to the website that says "Buy Now" blah blah blah. And Theory11 has a new deck, Ta Daa!