Is it possible to purchase (buy) an Android app from the Google Play store web site without causing that app to be automatically installed on a device?
As far as I know there is no way to buy an app without selecting a device and triggering automatic installation on that device. If the device is offline as far as I know the installation will be performed the next time it comes online.
The only workaround would be to add the Google account on a device, enable Airplane mode, remove the account and reboot the device. Then the device is still listed in Google Play Store for a few months and can be used to buy apps but they will never be installed as the devices comes never online again.
A similar option are "virtual devices" devices that do not exist in reality (or as emulator) but are registered in PlayStore. App download tools like Raccoon are capable of creating such virtual devices. But if those devices are not used for a time they also loose functionality.