Summary

Themes purchased through the Shopify theme store are only licensed to one account. A theme license can be transferred from one Shopify account to by contacting Shopify support.
​
If you don't have an existing license you will have to purchase a new theme license from the Shopify theme store. By failing to obtain a valid theme license your putting your store at risk of being deleted by Shopify without notice.

Purchasing a theme from a third-party marketplace will not give you access to a valid theme license. To avoid putting your account at risk all themes should be purchased through the Shopify theme store.