All sales tracking information for your products on the main Minted site and in your Minted Artist Store are in the My Reports section of your Design Studio (aka your Minted artist profile). Just log in to Minted, go to your Minted Profile, and click "My Reports" in the upper-right hand corner. There you can see your sales by product, by month, and by week. Be sure to explore the various tabs along the top to see more information about where your customers are (Sales by Geography) and how your sales compare to your challenge scores (Sales & Scores by Product).
For pieces that won in a Minted Challenge, you will not be able to tell which sales originated from the main site vs. from your store.
You can track sales of all the pieces you have self-launched in My Reports by going to the "Sales & Scores By Product" tab. The "Category" column will allow you to tell the difference between a challenge winner and a self-launched piece. Self-launch categories are currently MGA (general self-launched art) and MKA (children's self-launched art).