To learn to sell, sell something you aren’t passionate about.
I’m passionate about technology, so it’s hard for me to move out of the ‘development’ stage until I think that my product is technologically perfect. AKA: It’s hard for me to ship because I’m not happy with the product.
I don’t particularly have passions about art or writing, so if I was painting a portrait or writing a novel, I probably wouldn’t agonize over refining my product so much. It would be much easier for me to move on to the sell phase.
From there, if a product sells or not is purely going to be based on your soft skills.