Couple of things that I picked up.
1. Go to the eyeglasses market if you are looking for frames. Bring your prescription. Frames are like 10 euro the lenses are about the same unless you need the really thin glass (if you are like -7.00 or something) and there are 4 floors of choice
http://www.thebeijingguide.com/shopping/eyeglasses.html
2. The silk market is packed full of fakes. The shirts arent bad. shoes are very poor quality but are ok. As for haggling, be very agressive verbally. If they ask for 600 yen, you should be getting them down to about 100. Jeans should cost about 80 yen, no more. we spent a long time in china and nowhere did we see markups like beijing. For example a guy tried to sell me a mao watch for 150 yen. I got it for 25 yen.
3. Tea is excellent. Again watch out for scammers, go into a proper tea shop and haggle as if you are in the silk market.
4. Panjiayuan antique market on sunday is good but most of the stuff is crap. They are very shrewd and its so popular that if you haggle properly they wont sell it to you because there are so many dumb tourists paying ridiculous prices, they know that they can get a better price from an unsuspecting tourist!!
The haggling is great craic, if you are in the mood for it. The pearls there are quite suspect. We went into a pearl market with loads of sellers and I couldnt see the difference. They put a torch under the pearl to show that its real (it aparently doesnt mark), but its very unconvincing.