According to the spelling in 27.51, I'd say that you're buying a card, and not "a draw". Thus, 27.21 doesn't come into play at all, since you don't have a "draw" to forego.
This interpretation might be a little to literal, though.

And it might not be what AH had in mind when writing the rules.
Anyways, why would you want to buy from an empty stack? After all, you don't decide whether you're going to buy until after you've received your own trade cards. Would anyone be desperate enough to void 18 treasury tokens like this just to increase their stock?