I believe this is how you should go
Web Design & Development – make sure your design is catchy and attractive as per your products. Also the functionality of website should be user friendly. There shouldn’t be any trouble for users.
Platform – Website should work in all major browsers as well as it should open fine in Smart Phones and Tablets. So responsive solution is very important now a days with desktop solution as more and more people are using smart phones and tablets now a days.
Ongoing Management/Support – Payment options should be as many as possible. Offer multiple shipping option, if you can offer FREE shipping then it will be great. Offer coupons/discounts to your customers. Provide online chat/support, deal inquiries in a quick time when you will start getting them. Return policy should be properly defined. Provide the quality service so you can get more customers.
OnSite SEO – Very important now a days in terms of Google. Make sure your website is SEO friendly. Use proper Keywords, URLs, H Tags, Alt Tags etc. Google for details if you don’t have any idea about it. Last but not least, quality content should be there on website.
Links Building – Build quality links for your website, post articles on quality websites. Also you should have a Blog on your website and keep posting and promoting your Blogs on regular basis. There are so many ways to do it which you can Google.
Social Media – Social media really changed the game now a days. It is very important to have a presence on all major social media sites like Facebook, Twitter, LinkedIn, Pinterest etc.
Offline Marketing – Mouth word is very important for any business i believe. Business Cards, Brochure are some cheap but affective ways to promote your eCommerce solution. If you have some budget then advertise it on Radio, Newspaper, Billboards and TV.
As per my experience, I believe all of the above mentioned are important for success of any eCommerce solution. You cannot ignore anything.