What is the ruling on selling and marketing wedding invitation cards, knowing what takes place at them, such as singing and other things?

For this issue, there are three scenarios: The first is that you know, or it is highly probable in your estimation, that the person to whom you sell these cards will use them for propagation (da'wah) to an action that is compliant with Islamic regulations. In this case, there is no harm in selling them to him. The second is that you know, or it is highly probable in your estimation, that the person to whom you sell these cards will use them for propagation to an action that is not compliant with Islamic regulations. In this case, it is not permissible for you to sell them to him, as it falls under cooperation in sin and aggression. The third is that you know neither this nor that. In this case, you should consider the general condition of most people. If the majority adhere to Islamic regulations, then it is permissible for you to sell them to him; otherwise, it is not.