There are a lot of online communities where you can ask for feedback. HackerNews may one. Because of the free feedback, such services may only work if there is more than just feedback. On the other hand, many companies have their own graphics and web designer, so they won't need feedback. I like your idea but as was said before, it would only work when you want to focus on special groups. You should think about offering just a few features for paying customers and the rest is for free. You could sell advertising on your website to cover the costs.