If your domain sells during the 60d lock period, the domain can either be 'pushed' to another account at the same registrar, or the buyer has to wait before being able to transfer the domain to another registrar with auth code.
You don't have to worry about that. It's never a problem. Any buyer can open a free account with your registrar (whoever it is) and get a domain name pushed to him. If they like the name the 60 day lock will not affect their opinion.
And later, once the lock is removed the client will move the domain anywhere he likes.