But the whole point of a bank doing something like this, is that they have the reserves needed to not have to buy/sell on the open market. They can take your "sold" token and give you the observed market rate for it; then put the tokens you gave them in their own holding account, listing them on the exchange for the price they're willing to pay, and wait. They're in no hurry to liquidate.

If a buyer then comes along who's willing to pay that price—and that price is higher than the current market price for the token—then the token goes up in value.