Specialty booksellers often do very well with a website, no matter how small the inventory. General booksellers with larger inventories, or with very good quality inventories will also do well with a website. Having a website gives you a somewhat more permanent and trustworthy appearance. Especially to customers who are still a bit cautious about ecommerce. Brick and mortar storefronts each have a style of their own.
Online booksellers can appear rather faceless to a customer browsing the databases.
A website gives you a chance to create a bit of the atmosphere of an open shop. It encourages repeat orders. The mailing lists can be used to announce catalogue updates and special sales to your customers.