Honestly speaking, I think at this point it's best not to charge, or at least not much (you can use a donation system). Your banners are nice, but need a lot more professionalism to be able to charge money for it. A few examples of ways you can improve them is simple color theory (some colors are too saturated, don't work well etc...), some of your images aren't cropped properly, the text seems out of place, etc... I don't mean to be a debbie-downer, just trying to help you improve is all. Just try and make your banners simpler and cleaner, and get some more banners under your belt before you start charging. You have potential though, keep it up.