Praise be to Allaah.
Yes, the basic principle is that transactions are permitted and allowed, and it is not permissible to describe anything as haraam except with evidence. The ruling on the cards that you make for festivals varies according to the festivals in question. If they are cards with greetings for the festivals of the kuffaar, such as Christmas, or for festivals that are bid’ah (innovation), such as the birthday of the Prophet (al-Mawlid) or al-Isra’ and al-Mi’raaj, or festivals that involve imitating the kuffaar, such as national holidays and birthdays, then it is not permissible for you to make them or sell them, and it is not permissible for anyone to buy them, because that implies approval of those festivals that go against Islam, and because it is cooperating in sin and transgression.
But if they are for Islamic festivals – and there are no Islamic festivals except Eid al-Fitr and Eid al-Adha – then it is permissible for you to make greetings cards for them, and it is permissible for you to sell them, so long as you make sure to use phrases that are prescribed or permissible according to sharee’ah, such as “May Allaah accept from us and from you” and so on.
And Allaah knows best.