What services can they offer that aren't elsewhere? If it's unique, it won't actually be that effective. It can only be scoped to the instance.
Instagram isn't a comparison because it works entirely differently. By even saying "like Instagram" belies a lack of understanding of the topic.
It's true all instances are not equal. But your extension theory is not very strong. Nor is your instance-for-sale concept. They gain nothing from buying an instance. Users don't matter. They're just as reachable as another federated instance.