I just got GarageSale 9 to see the license file from my old GarageSale 6 which I purchased from the Apple App Store years ago. In case anyone else has the issue, here is how I did it.
Install old app store app from Apple App Store purchased section
Copy GS6_AppStore.rcpt file from ~/Library/Containers/com.iwascoding.garagesale7/Data/Library/Application Support/GarageSale to ~/Library/Containers/com.iwascoding.garagesale9/Data/Library/Application Support/GarageSale
Launch Garage Sale 9 and now when you license it will see it and charge 19.99
The license tool is no longer maintained so YMMV and it will prob break at some point.
Thanks to the great support team at iwascoding for helping me.