Is it permissible to sell old, unused Twitter accounts that are linked to an email address which has become my property, knowing that they have been abandoned for many years and contain nothing that would harm their original owners?

If an account is abandoned, and there are indications that its owner no longer needs it and will not be harmed by its disposal, then there appears to be no impediment to owning it and disposing of it through sale or otherwise. This falls under the issue of the permissibility of acquiring something that people discard due to their disinterest in it, as Islamic law considers circumstantial evidence and signs in determining rulings.