I'm completely confused with your verisions (non) management.
1) Why do I have to check all your messages in the forum to discover that you have released version 5.2?
Can't you send an email to your paying customers notifying about a new version and providing its new features and bugs solutions?
2) I purchased version 5.1 for 10 sites and got a link to download that version and the following upgrades withing version 5.
Where do I find the link to the 5.2?
When using the link to 5.1 I'm receiving a file with the same name as the one I got for 5.1.
How do I know that I have downloaded the 5.2?
In conclusion, I think that the handling of those versions is a mess and by being more organized, you will avoid a lot of questions and problems with your customers.
The quality of your product is not only in the code but also in the service you should provide to your customers.