I learned this term in drug rehab. Some people just cant stop getting high and they run out of crack, they are addicted so they search on the carpets to find little pieces just to get that high. Thus the term "carpet shoppers" is know in an addict's world.
John- "I just really need some crack right now so i'm looking for pieces i could have dropped!"
Emily - "Oh John, you are such a carpet shopper!!!"