A good ecommerce site to get you started in your new second hand book selling business seems to be the EKM Power Shop. You can find their website at https://www.ekmpowershop.com/ecommerce/website. By using their service, you will be able to design your very own ecommerce site that will look great and will give you a truly professional edge as an online second hand book seller.
This site will provide you with all that you will require to successfully sell your books online. There is a UK support team that is able to help you along the way and when issues arise.
Failing that – you could look into some of the more common e-commerce solutions out there such as the e-commerce arm of WordPress – WooCommerce. Similar to WordPress but varying in one or two features and cost is Shopify – both are great platforms for any kind of selling and not just restricted to books and both give you options to control your own inventory.