Imagine you own an orange tree, and you want to sell the oranges. If you run out and try to sell oranges, people will low ball you. What you really hope for is people wanting to buy oranges, and you try to sell then as quickly as they ask for then, or a little slower in order to drive the price up per orange due to demand making people offer more.
This is like trump owning x number of oranges, and saying they’re worth a certain amount per orange, so it is worth that amount times x. In reality, he only has shares he can sell, and there is the assumption he can sell them.