This is hard to answer in general, without looking at specific case of what sale of license implies for specific product.
For general GPL compliance note that what is typically licensed is not code. You just get code provided. What license buys you are usually things like access to support and updates. GPL doesn’t regulate those.
So yes, you can buy license for 1 site and install it on a hundred of sites. But then vendor will detect it and will drop their obligation to support it and give you access to updates.