I hope I won't trigger you with my personal thoughts here, but only measuring success by sales is the most effective way to become depressed, burned out and lose faith in yourself. For almost 15 years I've tried to figure out what sells the best and why. After 15 years I am still almost completely in the dark. Some things just work and catch the wave of inspiration that's going through the aether at the moment. From where I see it, your only job as a sound designer is to make the best possible art you can. What happens after are things that are way beyond your sphere of influence.
Just do the best sounds you can. That's all you can do really.
