I think this is indicative of the biggest problem we have had with social media: there is no legalism here, just "codes of conduct" that companies and users both willfully ignore. If your handle gets sold by some facebook employee to a rich kid in LA, what recourse do you have? I don't know what laws this would break (maybe some broad definition of fraud? I Am Not A Lawyer) so it's not like this person has a slam dun…
If the new owner is imposter trying to be the old owner, it would be identity theft and fraud. If it's just giving the handle to someone else there is nothing. If you don't pay for a account, you don't usually own it unless there is something I'm missing. If you want to own your identity in the internet, get a website you own and don't rely on shady social-media networks that give you something free.
Most registrars have decent trademark policies but again it's the internet. Just because you have a trademark in your country doesn't mean someone else doesn't elsewhere or that the registrar is going to care 5 years from now when the ToS change on renewal.
If you want to be securely verifiable on the internet you're better off being searchable and signing your content. The hard part is finding people that care enough to verify who you are in a decentralized manner for that to matter though.