It's boring to frame Mastodon as a Twitter usurper and much more interesting to think of it as an experiment for techno-socialism.
While users don't "buy" Twitter, Twitter sells user data to companies looking to make a profit off of users. People who have the means to create or support Mastodon instances provide the "service" for free. $ and tech skills get re-distributed to a wider community. Nothing and nobody is bought and sold.