What is the ruling on purchasing Waqf land and building a residence upon it, and is it permissible to sell it or reside in it?

The basic principle is that an endowment (waqf) cannot be sold, gifted, or inherited, based on the agreed-upon hadith (definition) of Umar: "Its origin is not to be sold or bought, nor inherited or gifted." An exception to this is when necessity or a recognized benefit dictates its sale or exchange, such as when it is no longer suitable or there is fear for its loss. In such a case, its custodian is permitted to sell it for the benefit. As for the land whose usufruct you purchased, you have the right to utilize it or sell its usufruct.