On the website Willhaben you can find private advertisements under the “Marktplatz” tab. You can find everything from bicycles to computers.
The website Geizhals is a price search engine, which will show you where in Austria you can get a certain product at the lowest price.
AtPreisjäger you can always find current bargains.
At Reddit you can find good tips and recommendations.
The simplest way to use your mobile phone in Klagenfurt is by buying a pay-as-you-go SIM card from one of the Austrian mobile phone operators. Pay-as-you-go cards are advantageous because you are not bound to a mobile phone company for a certain period. More information you can get at the Austrian mobile phone companies.
Other cheap options are offered by various video telephone providers.