Is it permissible, in Islamic law, to steal sand from the sea and sell it, given that the state prohibits it and there are other places to sell it?

The guardian (Wali al-Amr) must prevent individuals' actions that harm the public, even if those actions were originally permissible, based on the principle of "No harm [is to be inflicted] and no [reciprocal] harm [is to be inflicted]." So, if the state prohibits taking sand from the sea due to the harm it causes, then it is obligatory to listen and obey, because this is an act of "Ma'ruf" (good/righteousness) in which obedience to the guardian is required, and whoever violates this will be punished.