Can you resell access to your UpdraftVault account?

Yes. You are welcome to set up your clients’ websites to back up to your UpdraftVault account and charge them whatever you like. After one-time authorisation using your teamupdraft.com login, your login details are never needed again and are not stored in the UpdraftVault settings on your clients’ websites (in its place is a unique token with no other usefulness).