Is it really a lifetime service, or is that just how you're advertising it to get the sale? It is $20 a month for a normal premium account, so unless you're reselling hacked accounts the maths simply doesn't add up.
It's legit .It's my account. Don't tell if you don't know anything
It's not legit - I can do simple maths, even if you can't. Somebody, probably not you, is paying for an amount of authorized users on Grammarly. It is re-occurring and not a single payment. You are breaking the Grammarly terms of service by redistributing access to one or more of those accounts.