Every time that you have an isola rain bottle in the flea market, buy it. During the time that it rains bugs and rare/uncommon collectibles appear. Make sure to have your little people ready to collect! Even if most of the collectibles that appear you have to sell on line, they will most likely not be common. Another hint is do not sell your collection to the man that comes to buy it if it is unfinished. It will start all over in your achievements section. It can be very hard to get some collectibles back. Don't sell then to him if you want to achieve those achievements. Don't worry because the man will come back....even if it takes four generations. I have had him come back about five times in my fourteen generations of people.