Assalamu alaykumIn my place the Government Controls the Sand available on River bank and the government sells it to those who need it for a high price. But there are some other people who steal the sand from river bank and sell it for a low price. Most people in my place use the sand sold by these thieves. My question is, is it permissible to sell other construction materials such as cement to people who use stolen Sand for building homes or offices etc.,

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 no harm in selling cement to the people whom you mentioned in the question. Even if we presume that they are thieves [i.e. they steal sand], then their theft does not affect the permissibility of dealing with them, either by selling to them or buying from them. It is permissible to sell cement to them and other materials, and the sin of stealing falls upon them.

Allah knows best.