You can add a goal to your Analytics account for each sale that is processed through your Rocketspark website.
You can add a goal to capture the traffic that is passing through your checkout finish page URL (also called the checkout success page). Customers will only go to this URL once they have completed the purchase process: /shop/checkout/finish/
For example: www.mywebsiteaddress.com/shop/checkout/finish/
By using this URL in your Google Analytics goal you will capture the customers that have purchased an item through your site. To add a goal to your Analytics account, have a look at Google's help guide here: Create, edit, and share goals in Google Analytics.
You can also add specific conversion tracking to your checkout finish page, which is useful when using paid digital advertising such as Adwords. To do this, add the code to the Checkout Success tracking code section, in your Store Preferences: Conversion tracking - adding an ecommerce checkout success tracking code.