Windows 10 Mobile Store app has received a new update on Release Preview Ring. This update enables a feature that was already available on PC.
You can now buy Xbox games on your phone using the store app. The option of buying games was greyed out on Mobile Store app before this update but you can do it now.
As per the users who have received this update on their devices, the updated app version is 11709.1001.7.0 and this update is only available on Release Preview Ring so far.
It also seems that one can also buy those games that can’t be played everywhere. Do let us know in comments whether you have received this update or not.