do you want real or secondhand blackberry? you can check on "taobao" find some good reputation store in beijing, and go  to their physcial store in person. i think you could find some good BB.

there is link for example:   http://xc-blackberry.taobao.com/http://xc-blackberry.taobao.com/

There's an electronics market near Maizidian on Zaoying Nanli Road, not far from lucky street. It's called 258 Electronic Market. They have a whole floor dedicated to selling second hand phones. Obviously, it's always hard to tell at these places whether all the phones are genuine, but I've bought a second hand digital camera there and have had no problems. A lot of people seem to go there to trade in their old phones for new ones.

http://www.echinacities.com/beijing/listing/beijing-258-electronic-market.html