But the rules on almost all sites is that they don’t have value - the terms and conditions forbid you from trading usernames.
Like I say, they can’t have it both ways. Either they have no value and trading is against the terms, or they do have value and can be traded, in which case the website has a duty towards the user as the “bank” where the valuable item is kept. Furthermore, the higher the price Twitter are looking to sell usernames for, the more reasonable the claim against them becomes. $50,000 is a significant amount, one which a claim could reasonably be made for.
On a proprietary website like Twitter nothing belongs to the user.
Not true. If I make a post on Twitter, that post is my intellectual property. Twitter might claim extensive rights to user posts, as they are on their website and their terms and conditions claim such rights, but the user is still the owner.
Whether or not Twitter can even hold onto all of the rights their terms claim is also tenuous, as there is an argument that consideration (ie payment) should be given in return for those rights. Using the website is not really consideration, as the website is free to use regardless of whether you post content to it.