I buy a lottery ticket from time to time, husband and I do it for fun. I am not a gambling addict, nor do I sin by doing it. I am 100% sure my heart is clean about it or I wouldn't be doing it. We buy a ticket or two from time to time for entertainment. It's when people get greedy, reckless and obsessive that lottery tickets become sin (it's what proceeds from the heart that is sin).
Something is a sin if it has rule over you. Someone that has a glass of wine from time to time is not a sinner. If they drink all the time, and give themselves to drink, it is a sin, because it has rule over them, instead of God.
So while cigarettes are very unhealthy, I would still look upon this with a bit more love. People in prison or other situations, do you know what a cigarette means to them? Since some people are bulimic or obese because they eat too much, should people also stop selling food? Even the best things can be abused, lest cigarettes.
I would say, pray to Jesus and hear what HE has to say about it. If you are convicted still, it's better to change your workplace than constantly be in conflict. I'd just make sure first that it's really JESUS having problem with this, because sometimes people have convictions that God did not place there, these come from the weakness of faith. Some people, per example, believe anything but vocal music in worship is sin. Or they believe must eat a certain diet to be saved.