It would be permitted (halal) to sell the various types of contraceptives including the morning-after pill, as they have legally permissible uses. The morning-after pill (as explained earlier) can be used in cases of medical need; hence, it would be permitted to sell it. It is analogous to selling a knife, in that one may use it to cut a fruit, but unfortunately it could be used to stab someone.
Thereafter, if it does end up being used unlawfully, the seller will not share the sin or blame, because that unlawful action was purely through the deliberate willful action of the purchaser, not because of the one selling. One does not even need to ask or investigate about why it is being used. Assume it is a lawful use.
However, if in a particular case, the seller was certain of it being used unlawfully, it would best be to avoid selling it to that particular individual.