[JimDabell]

This isn’t quite correct. Vendors can and do verify Mac App Store purchases with applications deployed outside the Mac App Store. The installed bundle contains a receipt you can validate yourself. I think there is a gap where if you delete the application, then Apple removes it from the Mac App Store, you can’t reinstall it to get the receipt, but that’s a bit of an edge case; the majority of customers will be “upgrading” from the Mac App Store version to a newer non-Mac App Store version in a situation like this.

Also, if you have user accounts and plan for it in advance, you should upload the receipt to your server and associate it with a user account, which makes some things like this a lot easier.