I am CDN and I can't go to the USA nats but, here is a list of things I would want to buy:
Deckable Cards (Claydols, Uxie Lv.X, Trainers, etc.)
Cheap Packs ($3.00 is a fair price)
Sleeves (Variety)
Dice (For people without dice)
Theme Decks (for players who need a theme deck to play the game)
Set Binders (SF Binder, PL Binder, etc.)
Energy Drinks (idk if the vendor is allowed seeling food/drink but, any Nats is exhausting)