Is it permissible to sell decoration for a wedding that involves the mixing of both genders?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

If someone knows that the buyer of those items will use them in a party that will involve what is forbidden, such as the mixing of genders or music and the like, then it is not permissible to sell or lease to him what will help him in that.

Mataalib Uli An-Nuha fee Sharh Ghaayat Al-Muntaha, which is one of the books of the Hanbali School, reads:

"It is not valid to lease a house that will be turned into a church, monastery, synagogue, or a house of fire for the worship of the Magi or for the sale of wine and gambling because all this is helping in sin, while Allaah says (what means): {…but do not cooperate in sin and aggression.} [Quran 5:2] This also applies to when the house is rented in order to play the flute and sing as well as anything else that Allaah has forbidden. This goes regardless of whether this forbidden matter is conditioned in the contract, such as the tenant stipulating this in the contract, or when this is known by some indications, because it is a forbidden act, so it is not permissible to rent it based on this, such as a person leasing his slave so that he would commit immorality (homosexuality)."

Sharh Az-Zarqaani li-Mukhtasar Khaleel, which is one of the books of the Maaliki School, when mentioning the types of leasing that are forbidden, reads: 

"Or renting out a house or a land to be used as a church, a house of fire [for the worship of the Magi], a shop for the sale of wine or for pressing it, or a gathering place for the dissolute people, the same as the ruling on selling it for such purposes (because it is a kind of assistance)."

On the other hand, if one knows that the tenant will not use it for a forbidden ceremony or does not know whether or not it will be used for such a thing, then there is no harm for selling it to him.

Allaah knows best.