It doesn't have to be anything different than buying items under a bunch of categories.
What I specialize in, is building systems with database structure for such websites, from ground up.
So you will have users, roles, permissions, items or products, categories. If these items have related data such as images, tags associated with items/products. Then I write custom queries that will speed up the backend.
The front end, I use minification techniques to reduce JS memory. This way you have a very flexible, robust system. Of course, this needs as much time needed than a ready made solution. If you are interested let me know. All successful systems are built this way. I have 14 years of experience doing this.