ThemeStore is a WordPress theme shop that produce a variety of themes. They sell memberships which entitle the buyer to all of the current and future releases while membership is active. They offer a personal plan (use the themes on your own sites) and a developer plan (use the themes on client sites). Both plans allow you to use their themes on multiple websites. All of their themes use valid code, have SEO options and include documentation.
Premium WordPress theme sellers typically include support and upgrades with theme purchases. Visit ThemeStore's website for complete details including the most up-to-date pricing.