Yes, that's a good question.
For now, I have just created the categories and filled them once, but I did not go in the process of updating them.
The Top Sellers are the 4 all-time most sold item, taken from the Excel file (through a Dynamic Cross table, if it is the English name). I guess I should update the list every month (not sure how often it changes)
For the New category, I arbitrary decided a date and added all the recent models after this date, but I never removed anything yet.
I supposed it should be "all the models uploaded in the last 30 days" or something: this is not someting easy to do.
That's why having those categories updated by Shapeways would be handy.