-in Via del Governo Vecchio (close to Piazza Navona)you'll find a lot of nice vintage stores, for clothes and jewellery..also some cute shops with mixed designer brands...
-in Via del Boschetto (close to Termini station/via Nazionale)you'll find other secondhand shops.Pulp (really cheap new designers) god save the look (classy, good vintage) and Idra (clubwear, second hand and gadgets) and some more shops, always in the Monti-area...
-monday-saturday it's a quite big inside market in Via Sannio (Metro:S.Giovanni) from early morning until lunchtime. Some good vintage (the lady with the seventies-dresses in the end!), fur, bags, but also a lot of crap like Carhartt or Londsdale or Vuitton-copies..
-On sundays it's a small market in viale Flaminia on a backyard, like a garage-sale..my friend always finds original Chanel bags or Fendi shoes for 5 euros..It's open all day long..