In 7 Wonders Duel, one routinely purchases resources from the bank to build a Building. This is covered by these two rules:
Some cards have a resource cost. To construct them, you must produce the corresponding resources AND/OR buy them from the bank via the trading rules....
Often, you will want to construct a Building while you’re missing one or more required resources. In that case, you can always purchase the missing resources from the bank....
EDIT (to add a rules statement regarding a comment):
The first sentence of the Overview & Object of the Game section
In 7 Wonders Duel, each player is leading a civilization and will construct Buildings and Wonders. All of the Buildings constructed by a player together with their Wonders are called a "city". ... Each Age card represents a Building.
Then in the Game Elements ** section, the **Guild and Age cards subsection:
In 7 Wonders Duel, all of the Age and Guild cards represent Buildings. The Building cards all consist of a name, and effect and a construction cost.
Wonders are described in a separate subsection:
Each large card represents a Wonder from the Age of Antiquity. Each Wonder consists of a name, a construction cost, and an effect.
So, the rules are very clear in distinguishing between a Wonder and a Building in the definitions of the Game Elements. (end of EDIT)
Notice that the Trading specifically mentions Buildings and not Wonders. Is it the proper interpretation that Wonders cannot be built by Trading resources with the bank since Wonders aren't stated in this rule? Or is it allowed?
(For comparison, in regular 7 Wonders, the group I play with routinely purchase resources from neighbors in order to build Wonders.)