Doesn’t matter what price tag you throw on it, $5, $30k, $300k - Social Media accounts CANNOT legally be sold. Such is the case with Instagram, Twitter and probably every other Social Media Platform out there.
As soon as you buy a Social Media Account, you are putting a lot of trust into the seller, not usually an issue with anyone with reputation. However, should they decide to take that account back from you, there is absolutely nothing you can do, and you especially could not sue them or take legal action against them. I’m not a lawyer, and you’re definitely not a lawyer, it would be interesting if anyone could actually dig up a news article or more information relating to this.
I’m surprised it’s not common sense for you to think that if it’s against Twitter’s ToS to sell accounts, that would mean any kind of purchase of accounts would be void of legal action, and should only be done so at your own risk.