While I don’t enjoy going to a second-hand store nearly as much as wandering through an outdoor flea market, Prague is no Berlin or Amsterdam in this regard, and the options are limited. There used to be a market at Náměstí míru on Sundays, but I haven’t seen it in ages (although you can try the flea market at Tylovo náměstí near I.P. Pavlova).
Consequently, whenever I want to satisfy my consumerist cravings and buy something just for the hell of it, without spending a fortune, I usually go to the Prague Thrift Store.
A great shop just by the Šumavská tram stop, Prague Thrift Store is a great place for anything you might need, whether you just moved into an empty apartment and need a coffee mug or a plate, feel you want to update your wardrobe with that new spring jacket or you simply ran out of things to read. The staff is always polite and helpful, and they carry everything from clothes and shoes to household goods and furniture. They also have a wide range of books, mainly in English, since a large part of the shop’s clientele are expats living in Prague (expect to find a lot of English textbooks, guidebooks and maps). They also have a bunch of different sales and discounts each month so check their Facebook page regularly, or if you are a student, like myself, stop by the thrift store on any given Friday for a special 50% discount on all clothing items.