Not necessarily. You would do SEO to get more people to your website. To get more sales you need to work on your conversions. Conversions work is highly dependent on: usability, site design and a lot of testing.

In short, you need SEO to get people to your website and then you need to work on your conversions to get people to buy. If you have no visitors to your website you have no means to test your conversions.

This means that you would first need SEO, then testing for conversions. This also gives you some time to tweak the website. A very good website that is built by professionals with real world experience would give you a website with usability built-in. Thereafter you would only need tweaking for your unique business requirements to increase conversions, rather than worry about how many sales you lost in the first place.

---