When I set forth on this path as a writer. I swore a credo to myself. A golden rule which I shall never break, even if my e-books get returned, as some have.
My credo is "The work shall live or die by my hand." What this means is that my books shall thrive or die because of my writing.
I realise the trend is to get a star to endorse the book and thereby enhance the sales. But, can you look yourself in the mirror and hand on heart say. "My writing is the reason for the books success." Perhaps, you are of the school who say "A sale is a sale," and have no qualms how you get it.
My books will never sell well -- for various reasons -- but, I take pride in knowing success or failure, it was my writing which was the reason.