Hello. My question is that Im not sure the domain has fully transfered to me, although I have it in my account, but still - the seller name is in the whois details. Does it means he can access my domain? or it just some info about who sold me the domain?
As the domain name owner, you should have an account where you can edit the domain owner info. If you have the account, I would change the info, otherwise ask the seller to provide you with the access to the account.
Please log in to your account and go to your account settings and there you can edit the information to yours. valla! just as that simple. Is mydomain the new registrar or the domain came from mydomain?
Contact the domain registrar if you are unable to fix the issue through account settings. Normally there shouldn't be an issue.
it always takes time before the whois will be updated. I have been selling domains and transferred some of them to my buyer but some of them retains in my name
That is not ideal, it is better to contact the registrar and let them know your challenges. If they could not help, then it is better to start moving you domain out of such registrar. Cheers.