Generally, if sales is hard, it's also because of the product development. Don't blame all of it on him, your product is not as amazing as you might think.
My main takeaway from it was don't assume that you and your co-founder are on the same wavelength - write down explicitly what you are agreeing on, and be concrete with figures e.g. "I'm willing to work 8 hours per day, 6 days per week", or "I'm happy to consider this successful if I make $100,000/year" etc
http://vimeo.com/11491691
http://danieltenner.com/posts/0005-starting-up-with-a-friend...
Some thoughts:
Generally, if sales is hard, it's also because of the product development. Don't blame all of it on him, your product is not as amazing as you might think.