They're probably just reselling the cards. No need for anyone to buy digital assets from Apple.
If you want to save a bit of money, you can find gift cards for all sorts of gift card on eBay for a bit of a discount. Lots of this activity is legitimate, with the cards being sold by people who got the cards as a gift but would rather have a somewhat smaller amount of cash. Scammers can just hop right in to this market.
As long as they are a token that is sufficiently expensive to get hold of, I don't see why an economy can't work even if nobody ever spends them with Apple. It'd be like an economy based on gold coins in which nobody ever melts the coins down for their value as raw gold.
Relative to cash or gold, the token also has the not insignificant problem that it's time limited and can be secretly spent or voided, which should result in the card being substantially discounted (and checked for validity with Apple) each time it changes hands. It's also officially "not for resale"...