If I am working in a supermarket and I am just selling but I am not the owner of the business, does selling a pack of cigarettes mean I am sinning?.
Cigarettes are haraam and it is not permissible to smoke them or sell them or help with them in any way whatsoever.
With regard to the prohibition on smoking them, that is because of the real harm that they cause, in addition to other reasons such as wastefulness and extravagance. See question no. 10922.
With regard to the prohibition on selling them, that is because when Allaah forbids a thing He also forbids its price, so if it is haraam to smoke cigarettes it is also haraam to sell them, because selling them helps the smoker in this sin.
The Prophet (peace and blessings of Allaah be upon him) said: “May Allaah curse the Jews. Fat was forbidden to them so they sold it and consumed its price, but when Allaah forbids a thing He also forbids its price.” Narrated by Ahmad and Abu Dawood (3026); classed as saheeh by al-Albaani in Saheeh al-Jaami’ no. 5107.
With regard to the prohibition on helping with that, that is because of the verse in which Allaah says (Interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression”
And because of the evidence which shows that it is obligatory to denounce evil, and criticizes those who remain silent about it or approve it, so how about the one who helps with it!
It is not permissible for you to sell cigarettes whether you are the owner of the store or a worker. You should advise the owner of the store and explain to him that you will never sell these cigarettes so as to be free of sin. See also question no. 10204.
We ask Allaah to guide us and you.
And Allaah knows best.