Normally I would put some code on the "Transaction success" page of a checkout process which would mark in Google Analytics that a conversion occurred, but since Spark goes straight to the dashboard page after transaction I can't tell how to differentiate between a purchase and normal login.
A conversion takes place when a tourist to your site takes an achievement you care about and "converts" to a buyer. This could be through filling out a form, completing a purchase, or by only showing a high level of meeting with your site https://www.nx3corp.com/campaign-management.html