I don’t think selling accounts makes hacking accounts profitable. If you were to hack my account, and it was not possible to sell my account, you could scam someone with my account, which IMO is worse than you selling my account because if you sold my account, I could argue to the buyer their lack of a signed message is an indication they did not do proper due diligence and they were scammed by someone else, however if you were scamming from my account directly that argument would not work.

Further the reason accounts have value is because they can generate income. Using the above scenario, if you cannot sell my account after hacking it, you could use it to generate income, likes