It is not permissible to sell cigarettes even if you are working in the store
Question

                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?.
Praise be to Allah.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”

    [al-Maa’idah 5:2]
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.