It is permissible for you to sell condoms and other contraceptives, even if those buying may not be married. In a Muslim society, it would be the responsibility of the government to regulate such matters in ways that promote the general good. In non-Muslim countries, one should encourage, if one can, that such matters be dealt with in positive ways that would promote the general good.
The general principle is that if the service or product you are providing has a permissible primary use, then it is permissible for you to offer it--and you are not sinful if the buyer chooses to use it in impermissible ways.
However, when one has reason to think that what one is providing may be used in impermissible ways, it would be praiseworthy to avoid providing it--without this being obligatory, and with due consideration of circumstances and consequences.