If you are certain that your Store is setup correctly already and the buying button is still not showing, you have most likely only enabled Store at an account-wide level. You still need to turn on Store at a collection level. Under the Collection tab, select the collection that you would like to enable selling. Under the Setting tab, click on Store and change the Store Status to ON.
Note that collections created before your Store is set up will have Store OFF by default. You can turn it on by going to each collection to update the Store Settings.
How do I turn Store ON?