All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is nothing wrong with selling mattresses to non-Muslims, and the possibility that they may commit a sin on it does not affect the transaction. It is not forbidden to sell to them, and the seller is not sinful because there is no direct aid or intention on what is forbidden. The mattress is not used specifically for what you mentioned. The rule is that what is used for permissible and forbidden purposes can be sold unless the seller knows or predominantly thinks that the purchaser will most likely use the item to do something forbidden with it, in which case it is forbidden to sell it to him. The Kuwaiti Encyclopedia of Fiqh regarding the sale of grapes although wine may be made from it: “The Majority of the scholars stipulated in forbidding this sale: that the seller knows that the buyer intends to make wine from the juice; if he does not know that he would use it in what is forbidden, then it is not disliked without any difference of opinion about this matter.
Allah Knows Best.