Praise be to Allah.
It is not permissible to sell haraam things such as cigarettes, pork and so on, because that is helping in sin and transgression, and because when Allah forbids a thing, He also forbids its price. The cashier is the one who handles the sale directly, so it is not permissible for him to sell haraam things, and he is sinning every time he sells a haraam item. His sin is no less than that of the one who promotes it and the one who carries it, because he is the one who conducts the actual sale and all other activities are connected to what he is doing.
And Allah knows best.