Are assignments legal in Los Angeles?

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Hello, 

I recently went to a RE meeting where they said that assignments and double closes are illegal in California (Los angeles), but I sort of don't believe it. Do you need to buy the property first, and then sell it? Or are assignments and double closes legal? I have my RE agent license, but it is not hung on any broker's wall. Also, when you buy from MLS or from a seller with a broker, how does wholesaling work?