What does it mean to "sell a song"

Alicia Keys: true salesperson

We occasionally hear the phrase: "She (or he) really knows how to sell that song".  What does that mean?
Any salesperson who understands what it means to really "sell a song" will understand what it means to truly sell a refrigerator or grinding tool or insurance policy.
It means that you believe so strongly in what you are saying (or singing) that you can convince your audience (or customer) he should buy your product (the song or the refrigerator).  A singer who cannot sell his song is boring; when you hear a song that is just being sung and not being sold, you turn it off.
This is the same in sales.  If you can't believe in what you're selling, your customer will be bored--will turn you off and you won't make the sale.
If you don't believe in what you're selling, you need to find another product or another profession.
If you do believe in what you're selling--if you believe it will change your customer's life, or his business, or the world, reach into your soul and SELL IT, just like a singer sells a song.