You can mark each item in your Collection as private (it will then not show in your Showcase). If you want to change the privacy of existing Collections go to Collection Management and use our bulk change feature there.
You can also hide the Total Estimated Value of your Collection, your Top Ten and Estimated Values on your not-private Collectibles. To do that go to the Account Settings on the top right of your screen -
Click on Account Settings and scroll to almost the bottom of the page where you can see these tick boxes -
When done, do not forget to click on the Save button -