Odds are you'll find a better price for certain similar items. Be sure to travel down Pandrossou St. to Monastiraki Sq. and then down Ermou St.
Sunday mornings things are usually less expensive and you'll find more Greeks shopping as well. On the other hand not everything will be open either. I was there this Sunday and it was a zoo with lots of people so if you don't like crowds try another day. And be wary of pick pockets of which there are a few preying on tourists and Greek alike. Sunday is more like junk day with a few exceptions and you can be shoulder to shoulder with all kinds of interesting people.
Some of the items for sale: Furs, jewelry, jeans, leather jackets, brass, shoes, rugs, surplus military, books, museum reproductions, sandals, scarves, records, old foreign books, food, icons, electronics, tools, musical instruments and on and on and on! Also see Abyssinia Sq.
For more Athens up scale shopping check out Greece's biggest Mall the Attika Mall only a block outside of Constitution (Syntagma) Square and there are sales in July and August to take advantage of as well. Additionally Kolonaki Square is the shoe mecca of Greece where many designer and famous brands are to be found.