lets just make one thing clear no one "owns" an account. All accounts are leased to you in a limited nature: in such nature the lease is non transferable to another. Therefore, it isnt really against TOS, but more so that the company has no obligation to honor the account with the new user. Proving that this happened is another thing entirely since ips are just locations of access and not physical users, so just an ip alone is not proof of a situation of transferring of user. However, multiple access points from different ips that are not consistent could help with providing evidence to support a claim that a user is not the owner.