What do you mean by selling the ticket? I am not sure I quite understand what you mean. If you mean what I said before, that he could have explained that if he did not sign the ticket, he would go to jail, then I understand. But from an enforcement standpoint, it is not necessary to sell anything.
What do you mean by his responsibility? If you mean that you believe that if he had used other tactics he could have potentially modified the suspects behavior, preventing the need to have to taser him, then I agree. But are you saying that the use-of-force was unjustified?
Common phrase in the LEO world is that you need to sell the ticket so that it is easier to stomach for the offending person. I learned this in the academy in 1992. By selling you explain: why you pulled them over, how fast they were going, why you are writing the ticket in general another tactic used is to let's say someone was going fifteen miles over the limit to only write them over for 1 to 5 mph so the ticket costs less for them. Further when you encounter somone who is being difficult that may mean that you have to go over things a couple of times to explain it more thoroughly. In the end if you write a ticket and they accept it then there is alot less paperwork, time, effort, etc into the encounter than if you say arrest someone or tazer, etc. In the end I imagine that this officer wishes he had handled this situation differntly.
As to the question of his responsibility it flows in line with selling the ticket or explaining the reason for the ticket. I think if the officer had done a better job of that then this encoutner might have been handled better.
As to the use of force being unjustified so many factors go into this and the officers perception of what the offender was going to do or the feeling that the officer perceived that it is hard to figure that out based just solo off this video.