Xi Dan is a very large shopping area and often frequented by real Beijing residents.  Here you can find all kinds of things and almost no "especially for tourists" fare.  The main focus of the area is 5 or 6 giant malls.  These are very old fashioned, Chinese style malls, similar to a US department store.  Most have 5 to 7 floors and sell the usual stuff you would expect.  Electronics and high-fashion are usually located towards the top floors.  If you're brave enough to buy a TV, DVD player or a Hi-fi, there is a good selection in the black glass mall with the large picture of a girl pictured in a later photo below.  Probably the mall with the worst selection of everything is the first one you come across (pictured below left). Leave it till the way back, or skip it altogether.