If you are in Athens of a Sunday, and are an early riser about 9-10 am head on down to Avissynias Sq. Its so called because in ancient times the slaves from Abissynia were quartered there.
On Sunday its an eclectic selection of items mostly, of particular meaning for people who live here but I bought a small brass lamp once and took it back to the US with me.
I had to get the plug changed but have never seen another like it either.
The rest of the week Abyssinia Sq. offers some similar activity too but with fewer crowds and is therefore preferable, if you are actually in the market for something. Just as always, shop here at your own risk and be skeptical as to vendors claims.
Avissynia square is easy to find from Monastiraki square. See photo above right. Go down the street (Hephaestou) (Vulcan) designated by the banner.
One may egress it from parallel Ermou Street as well. Sundays are busiest and beware pick-pockets. Things start early and wind down around 14:00
These photos will, if you click them give you a real good idea whether Abyssinia Sq is for you.