wow... Answering your question would be a book in itself.
Unless you're selling very niche specialty products your walking into the teeth of some very stiff competition. Your very first marketing consideration needs to be the site itself. I'd suggest doing a lot of research into what makes for a really good ecommerce site. You can do all the marketing you want but if your visitors don't like what they see when then get there you've lost the battle.
If your site is ready, or once it is, you might consider putting it up on the Website Review board for us to take a peek at.
Dave
