This is an accepted solution.
Welcome to Shopify
You can purchase theme but you can't use that theme for multiple stores as per my knowledge. You have to buy a theme for each store.
It's best to talk with theme author first about his product offer.
Hope this helps.
Nick here from Shopify.
Just want to chime in here and back up @dmwwebartisan's reply saying that is correct. Each store requires a valid theme purchase from the Shopify theme store.