Some possibilities to get used stuff on a fair price level.

ESN Vienna Sell and Buy  Sell or buy stuff you need or don't need anymore on Facebook.

Willhaben Here you can find nearly everything what comes to your mind!

Shpock This one is like an online flea market. There is also an app available!

Feschmarkt You can apply for a participation and then you can bring your old clothes and change them with others.