Personality and product -- the two sides of selling an artistic work, be it a painting, a book, a musical recording. Both should be promoted.
Of course, the product is paramount. It must be something that people actually want, that will promise some lasting value. Remind them of that. Tell them how it will useful, desirable, how it will change their lives!
But the buyer also wants to connect with the creator. Personality does sell. Or, at least, it attracts the buyers to the product. Would Lady Gaga sell as much without her persona? Although she seems reasonably talented (more than the average pop star, anyway) there are plenty of competent unknowns out there who can't attract an audience.
Let's face it, we -- whether singers, writers, actors, jugglers, whatever -- are in the entertainment business. So be entertaining. Promote yourself!
Remember, self-promotion doesn't necessarily have to be your real 'self.' No one cares who that is, except your mom. And you can probably fool her too.
Never forget, though, that it is ultimately the product that has to have substance. Never forget that it has to promoted equally. Product and personality; that's what sells.