Microsoft’s head of Xbox Phil Spencer has mentioned on Twitter that the company is “working on a solution” to allow purchasing of Xbox 360 games on the Xbox One.
— Phil Spencer (@XboxP3) December 8, 2015
Presently you can play a selection of Xbox 360 games on the Xbox One through backwards compatibility but can only buy Xbox 360 games through the online store or the 360 dashboard. While disc based backwards compatible games work by simply putting the disc in and downloading the game, Digital Xbox 360 games show up in your library to be downloaded, but cannot be purchased through the Xbox One interface. According to Spencer, that should change in the near future.