You will probably come across supply quests where you have to supply some of your neighbours' buildings, only to find that none of your neighbours need supplying! The best way around this is to physically ask them to stop supplying a few of their buildings. That way it will be left open for you to help out.
Gold Dust is pretty much the hardest thing to find in The Smurfs & Co. It can be so rare that you think none exists in the game at all! However, the way to get it is to smash rocks. Do this as much as possible and you will eventually come across the precious dust.