What are the conditions for waqf (endowment)? And is it permissible to sell it?

Regarding the sale of waqf (endowment)?
If its benefits are no longer available, it is sold. Some scholars said: If the sale is for what is better, it is permissible, in the sense of benefiting the one who established the endowment. So if it is in a place that is suitable, or its yield has decreased, and he sells it for what is better, there is no harm. However, the well-known view among scholars is: if its benefits are no longer available, or it is almost so, such as a neighborhood where the people have moved and the house no longer amounts to anything, and its rent is low, it is sold and another house is sought in a desirable location.
Q: If a man endows a building and says, "The rent is for you, but if I die and my children need it, they will take it"?
A: This is not an endowment; this is a donation of the rent.
Q: Are books considered part of knowledge?
A: Yes, the endowment of books is part of knowledge.
Q: But if he endows them in his house for his house?
A: It is not called an endowment unless he endows them. If he endows them for the heirs—for his family as inheritance—it is not called an endowment. But if they are benefited from, he is hoped to receive good.
Q: If he endows all of his wealth?
A: If he is healthy, it is better to leave some of it for himself, as Ka'b ibn Malik said: , meaning: leave for yourself something to sustain your condition, and endow what is easy; so that he does not become forced to be poor and beg from people. He should leave some of it for himself.