E-commerce Website w/ member logins - 200-300 products

We need the backend of a decently-sized (200-300 products) E-Commerce website coded and implemented within our front-end designs. You will not be designing the page, however, you will implement a shopping cart system into our HTML/CSS layouts. The shopping car needs to work like this: To actually purchase products from the 'E-store,' there will need to be a 'member login' section, but to simply view and access the products, there will be no level of validation. To clarify, if you are a non-member, you can ONLY view products and descriptions and photos. You HAVE to become a member to actually 'buy' something. Again, to clarify, the website will function the same(navigation, layout, amount of products, etc) for members/non-members, except that members will see the Shopping Cart function and have Shopping Cart privileges. Once inside the E-store, the member can save items in their shopping cart, add/remove items, and the other popular features in an e-commerce site. The site will also need to warn the user before he/she hits the back button, redirects to a different page, or attempts to exit out of the domain if there are any pending transactions/shopping cart items un-purchased. To set up an account/become a member, the user will need to either: A- work for -or- B- own a business with the interest in reselling the bed/bath products at wholesale(the price we will offer to members only). That being said, the user will need to have a 'Company Name' as part of the initial login registration for proper validation, along with a few more custom fields. Total custom fields for user to input at initial registration: 25-40, with 10-20 being required*. Once a user becomes registered, this will have to be linked to a database so the 'physical' store, has access to all the registered accounts. E-store will need to be easily updatable with either a CMS (custom built, 3rd party, template based, shopping cart based, etc, etc) which is preferred, or linked to an external XML file(or something of the sort) that can be easily updatable by someone with 'intermediate' web knowledge. As mentioned above, the E-store will have two-three hundred products. With 3 main Categories, and several sub-categories under each one. We will break down the navigation and all the subdirectories, you will be implementing it within the product line and within the E-store's database. The store will need a database that can automatically pull the appropriate information, and display the appropriate information when/if the user clicks a category. There will also need to be a 'search' function, like a typical 'google search' or a 'wordpress search' that displays the results within the designed layout. Each product will have a photo(or 2,3,4), detailed description, and a price. The company is in the bed/bath department, and is expanding quick. The product will be handed to you as basic CSS/HTML layouts. Preferred shopping carts may be suggested, but you will be responsible for building this from the ground up. If there is a template-based system that already allows for this-great. It's your job to implement it.