I went through it manually. The good thing is that the site lists 100 cards per page, so I simply had to scroll to the end of each page and see if it was 100 cards - if not, then I went back through the one page to see the missing number. If I hadn't been able to do it easily by eye, I would have done some copy/paste magic into excel and written a little formula to show me skips in the sequence.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).