Sadly, The Del Negro Grand Slams are the only decks of good cards that I can also easily find for sale, pre marked:
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Not that I think any of our players would swap out cards, but I'll not invite temptation.
I get it. I came across some of those sites when I was checking the internet to see if there were any cheaper sources of these cards. But as Matt pointed out, they are available for all major brands, not just the Grand Slams, and they come (re)sealed so it's hard to tell.
Not saying this to try to convince you or anything, but my understanding is that these marked cards are quite expensive. If I'm collecting ~$1K from the GB participants, I'm not paying 3-4x that (or more) out of my own pocket for marked cards.
If anyone gets fancy UV/IR/RFID-marked cards for ~$12/deck from GBs, turn around and sell them to cheaters for a big profit!