Your letter was more or less clean, without many grammatical errors, but I took the liberty to alter some things for greater readability. Overall, remember
the KISS rule— Keep It Short and Simple 
Dear Sir or Madam,
I am a customer of your department store and
I am writing to complain
about a product of your store that I found unsatisfactory.
Last Friday I bought a CD player
from your store. This morning I was listening to music, when
suddenly the CD player stopped working. I tried some other discs to check if something was wrong with the disc. Unfortunately, the player still failed to start. It is evident that the CD player
is faulty. As the
time since the purchase does not exceed one week, I would like you to replace my CD player with a new one.
I enclose the model number of the CD player for you to check, as well as my telephone number. Your prompt reply would be appreciated.
Yours sincerely,
Chengwei Liu