Explanation/Reference:
Configure app requests and SharePoint Store settings
Farm administrators can determine whether users can purchase apps from the SharePoint Store. This setting is at the web application scope. If users cannot purchase apps, they can still browse the SharePoint Store, and request an app. Farm administrators and the App Catalog site owner can view and respond to app requests.
To configure SharePoint Store settings
1. Verify that the user account that is performing this procedure is a member of the Farm Administrators group.
2. (A) In Central Administration, on the Apps page, in the SharePoint and Office Store section, click Configure Store Settings.
3. On the SharePoint Store Settings page, verify that the selected web application is the web application that you want to configure.
If you want to switch to a different web application, click the down arrow next to the web application URL to change to a different web application. (C)
4. To allow or prevent purchases, select an option for Should end users be able to get apps from the SharePoint Store?
Select Yes to allow users to purchase apps.

Select No to prevent purchases but allow users to request apps.

5. To allow or prevent apps for Office from the Office Store to be started when a user opens a document in the browser, select an option for Should apps for Office from the store be able to start when documents are opened in the browser?
Select Yes to allow apps for Office from the Office Store to start.

Select No to prevent apps for Office from the Office Store from starting.

6. Click OK.
Reference: Manage the App Catalog in SharePoint 2013