Question [Q] Buying 2nd hand smartphone in Vienna

Dear forum members,

My brother-in-law is in Vienna for his German course that will take 6 months then he is planning a graduate degree in Austria. We are trying to communicate via Viber but his current phone (which is my ex-Desire Z) is causing problems. Can you advice me a shop or a mall that he can buy a second-hand smartphone so we can use Viber and Skype easily?

Thanks in advance