Sali Pazari is a huge market on the Asian side of Turkey in the Kadiköy neighborhood. Different vendors show up here throughout the week. On Tuesday cheap clothes are for sale. The quality isn't great, but you can find anything at rock bottom prices.
Sunday is probably the best day to go. There is a unique assortment of furniture, jewelry, and antiques on sale -- things you won't be able to find anywhere else. You'll have to fight the crowds, but it's a fun shopping experience.
Location: Near Fehnerbache Stadium, Kadiköy