There are three possible scenarios with any product that one may be selling or buying:
1- The product can only be used in a Halal (lawful) purpose
2- The product can only be used in a Haram (unlawful) purpose
3- The product can either be used for Halal (lawful) or Haram (unlawful) purpose
The ruling of first kind of products is clear and simple: it is permissible to buy and sell such products under normal circumstances.
The ruling of second kind of products is also clear and simple: it is not permissible to buy or sell such products under any circumstances unless one is going to destroy it or completely turn it into something lawful or neutral.
The default ruling of third kind of product is that: it is permissible to buy or sell such products unless one is certain that the product will be used for Haram (unlawful) purpose. It would be impermissible to sell such product to someone whose known to use it only in wrong (unlawful) purpose.
The ruling concerning your question in particular is that: you are allowed to sell the prepaid card under normal circumstance but if you either know with definite knowledge that someone is going to use it for (unlawful) activity or have strong suspicion for the same then you would not be allowed to sell to that individual or party.
You are not required to issue a warning to every individual purchaser. However, it may be advised that you post a warning for buyers that may be visible to them at the time of purchase.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America