Regarding:
You can not switch licenses. Extension purchased to a specific store can not be licensed to another.
Then you need to state on MarketPlace on each extension page this parameter.
Currently you say: Single installation, is for use by single site or one copy of application using this extension.
I also suggest you rethink that. If I buy an extension that I think might work for one site, install it and find it is not suitable I want the ability to transfer it to another site that it will work for. If I buy it, and don't violate the single site rule I should have the opportunity to install it on any site I wish at any time. Just like every other software I buy from other vendors, it is transportable.