The Stock Market
The Stock Market is more liquid than some of the other equity markets because there are 'market makers' who are required to make sure trades happen. They are responsible for buying when there are no buyers and selling when there are no sellers. This is handled through a complex network of brokerage and clearing houses.
When you buy a stock, bond, mutual fund or other instrument from your broker it is usually held in 'street name' meaning no stock certificate is issued by the company in your name. You can elect to have them issued to you, in your name but to sell them, pledge them or use them in any way with any kind of option strategy you have to surrender them back to the broker to be put back into 'street name'.