There are many good articles for you to look at simply search for eCommerce site design or similar keywords.
Keep people, your buying customers, in mind because everything you do is to fulfill their desire:
- Don't get hung up too much on the platform. I've settled on Magento and there are many others. A WordPress plugin may be enough for a few specialized products. You can find a hosted solution if you don't want to deal with the store technology.
- Trafic matters. People must find your product!
- Presentation matters so use eye-catching graphics and be concise with descriptions.
- Include related products but don't overwhelm.
- Make it easy for people to link, like, email, and add your product to their wish-list because people don't often buy on their first sighting.
Take some time to learn about:
- analytics tools
- keywords and search engines
- product categories and descriptions
- page layout and hot spots
Here is a great place to start: Tripwiremagazine/8-seo-tips-for-ecommerce-websites
