Seller should bundle a license agreement in the package that allows his project to be used in commercial or free non-profit projects.
Scirra licenses that are issued on the store are covering non-profit learn only usage as a third-party (meaning you can't use it to resell the assets or make money with the game but only learn from the templates there atleast for the construct game+source examples), however that license between buyer and scirra doesn't cover the seller agreement to client who created and put the items in the store.
All other cases you have to contact the seller personally and ask him what is his stand on the usage of his assets.
This is a old problem, but is easy to solve, by simply contacting the seller.
Atleast that was the case for a few years, not sure if the store licensing has changed.