You can view items that you've purchased by going to the art library tab and clicking on the black star next to the search box. This will open up your personal art library. Near the top right of the popup, you'll see three options: Your Library, Your Purchases, Upload. If you click on "Your Purchases," you'll be taken to a listing of items that you've purchased in the marketplace. Items that you've purchased should also show up in the art library tab search. Using tags can help you find them more easily, but you can also use the yellow star next to the result in the search listing to "star" ones you'll want to use again. If you do the same search, those starred results should show up at the top.