And if you don't do it right things can get all screwed up in a hurry. California tried to do this with some really bad rules (power companies must by power in an unregulated spot market but sell into a regulated market for some number of years) in 2000 and ended up with the bankruptcy of PG&E, rolling blackouts, $40 billion lost by the state of California due to manipulated inflated prices, and the only successful recall of a California Governor in history. [1] for the gory details.

[1] https://en.wikipedia.org/wiki/California_electricity_crisis