The new subscription-based version of Quick Sale allows previous users of Quick Sale Pro to continue using the new version without a subscription.
To do this, just tap the 'Restore Previous Purchase' option when the Subscribe page appears.
If this is not working for you, it could be one of the following two cases:
The Apple ID configured on your device is not the same that you used to make the original purchase of Quick Sale.
You have the original version of Quick Sale upgradable to Pro with in-app purchases (discontinued)
If you have the original version of Quick Sale upgradable to Pro with in-app purchases, you may need to download a different app from the App Store. Please see the help page titled 'Can't Install or Update Quick Sale'.
To check or change the Apple ID configured on your iPad open the Settings app and tap iTunes & App Store.