OK, I did it at last! Here is what I did:
When my Sim reached an impressive reputation, he could not sell anything for about 2 or 3 days. Then I cancelled the consignment, saved the game and exited to Windows. It is not enough to exit to the main menu by the way. I also reset my Sim as well.
Then I entered into the game and consigned the items. True enough at around 5:00PM, my Sim could sell his items. Not only that, the impressive wordings appeared as bold letters!
Thus, I have finally truly completed the consignment challenge! Now, I am trying to get 15 gigs for my Sim and hope to achieve that!
Hope this piece of information is helpful to you guys!