If any company tried to follow all of the conflicting SEO practices that are advocated today, they would not only run out of time and money but they would end up creating things and taking them down every day.  There are thousands of "experts" who will tell you what and how to do it and thousands of companies that will take your money.
Here are my top ecommerce specific online marketing rules that we make sure our client's follow and have achieved great results.
1. Plan a campaign and then measure it.  Analytics are key to the success, especially because if you execute it well, you can actually measure the effectiveness of your online marketing $ down to the penny.
2. Focus on driving traffic first, conversions second.  If no one ever sees your site, you will never sell anything and if you are even a mid-sized retailer, your corporate brand isn't going to drive that much traffic.  Decide on your traffic driving strategy and spend on that.
3. Be really relevant.  Make sure your pages, titles, urls and tags are designed to win the search phrases that are relevant to what people are actually searching for when they are using natural search to find the products you are selling.  
4. Mine your network.  Spend money on marketing through people who bought or like your product, or are connected to your buyers, before you go out and spend money on wholly new customers.  You can do this through facebook and twitter but you can also do it through more traditional marketing channels like coupons and rewards programs for referrals through your current clients.
5. Focus on your high $, high margin products where you have the least competition.  If your are just starting your online store, make sure that you are focusing on the products you can actually win and if you win them, will make you some money.
6. Backlink.  Make sure to ask all of your suppliers and partners to link to your page and make sure your blogging and facebook campaigns link back to your site to build link juice.  
7. Page speed.  Make sure that at least your homepage runs fast.  Check it with google webmaster tools against the industry average.  Make sure you are in the top 50%.
8. Build key microsites.  Make sure they are relevant to the product, category or sub category you are trying to win, make sure the content is relevant to the key phrases you want to focus on and then make sure it's easy for the consumer to buy from that site.
9. Blog, twitter...  a lot and link it back to specific products, categories and landing pages.
10. Remember your purpose.  It's not to provide information or to provide a social media experience to your users.  It's to sell products at a reasonable profit so really take the time to set up each campaign well, tie it to good analytics and measure how well it works.  I can't tell you how many (even large sites) spend money and have no idea what effect their marketing dollars have on their top and bottom lines.