I want to contract a new community and sell sand, cement and bricks. But how to sign a contract (agreement) with the property. Pay attention to those details. Ask experts for advice.

Signed for nothing. Property can prevent others from selling these building materials in the community, but it can't prevent owners from buying building materials outside.

Do you think that by signing an agreement with the property management company, the property management company can stop the owners from buying materials from outside? That can only be said that you think too much.