Build Database Driven e-Commerce Website

I'm looking to build a site [login to view URL], a fairly simple site selling my work under various categories (Hats, sweaters, appliques, etc) and each item also having it's own page - with large photo and various other expandable subphotos photos with more details on the item. The database entry would have certain fields - with each field being directed to one (or more) of three areas of the site. Some content goes to a blog post (Main image, blog text, user data, price, etc), some to a Gallery Listing (Main Image, Gallery Text, price) and some to a Product Listing (Main image, supplemental images, price, user data, color options available (for hat, flower, trim etc) detailed description, etc). All products sorted by categories but also searchable by keywords. I'd like the backend easy to add new Categories, Available Colors Fields (with photos of that coloured yarn) as well as change product information and manage different users. Each item will have the opportunity to be 'user ranked'. They click the Heart and the more 'hearts' an item has, the more popular it is - thus the ability to rank products by 'most popular' instead of most recent, which is default. It will use a paypal shopping cart. I envision the site set up like www.thymematernity.com. I like the white, clean look. A slideshow of large images (we can check off a 'frontpageimage ID" if we want this image on the front page when entering in each image) as well as a sidebar (showing on every page in the site) showing featured blog post, text denoting any specials and promotions and latest gallery items. (Or when in the Gallery Page, the gallery items don't show, but shows the latest blog post and visa versa). For each gallery Category view, Product Listing and general pages, they all must have the same look and feel (with the same header, footer and sidebar) and each has it's own banner (within the body). These banners I'd like to change out seasonally or if I get better pictures. I am proficient in Photoshop so all I'd need to know are the dimensions for the banner and the just upload the file into the banners section and check off the radio button for "Hats Banner". I would like to simply add banners instead of renaming/replacing them as I may want to reload an old banner in a particular section (ie: A Christmas banner used each year) All banners will be identical except for the banner on the main page - see Thyme Maternity. I think the future of this site is letting others signup and sell their work for a monthly fee or commission on each item sold - so, either setting this backend bit up now - or at least leaving that so it can be expanded later without having to do a backend database redesign (See [login to view URL] for an example of multiple users selling their own crafts in a virtual craft fair). If adding this component is expensive we can leave it out - but I would like to know that the option is available to add this later, if we decide to go this route.