For an online store a good solution is to use an E-Commerce CMS. A bespoke online shop is really expensive and there are good E-Commerce CMS's available.
The best e-commerce CMS depends of what you need. If you have a small business, maybe is better a simple CMS like Open Cart or an e-commerce plugin for wordpress. If you need something big, Magento E-Commerce can be a good choice.
I'm a web developer and I develop small and big e-commerce websites. I use the CMS's Magento eCommerce, Open Cart or Wordpress.