Both μαγαζί and κατάστημα mean shop.
Μαγαζί is a word with Italian origin and is more informal.
-πωλείο comes from the verb πουλάω (older form is πωλώ) which means sell. -πωλείο is always the second part of compound words like κρεοπωλείο (butcher shop), παλαιοπωλείο (antique shop) etc.
