An exchange of money for a thing usually means that the buyer wants the thing more than they want the money, and the seller wants the money more than they want the thing, but in this case the seller is selling ... money?
Is, uh, the delivery date guaranteed?
It's not quite as simple as selling money, since the amount of money that the box will print depends on various factors like the difficulty, exchange rate, and delivery date. So it's really a question of whether the customer's risk analysis is better than the vendor's (which is doubtful given the information asymmetry; see also Goldman Sachs). And the delivery date is definitely not guaranteed; most vendors are shipping late.
In essence, they're selling a risky bet. It's like a lottery ticket, except more dependent on external factors. The buyers get to make a bet that might pay off, and the seller gets more concrete payment in exchange. It's sort of a physical embodiment of a financial instrument.