It would be useful to be able to view the marketplace through the mobile app. I would especially be interested in features similar to the eBay mobile app. Mainly ones, such as getting a notification on my phone when an item sells. This would allow me to potentially ship sold items more quickly.

Let me see only listings with actual photos of the item instead of stock photos.

Your pricing on some rare games makes no sense - for instance Bang-O NEW sealed copy is less than a loose price. Obviously this makes no sense and your system should recognize this. Don't use any price would be better - although I found examples offered on games on Amazon. This is a bad flaw, and really hurts the credability of your site.

Would you add Beckett integrated prices & half grades, please? Without, makes it difficult to sell on your market place.

BTW, Thank you for adding CGC. 

Add Categories for G1 Transformers, Studio 86 series Transformers, G.I.Joe, TMNT, NECA figures and so on. I’m really interested in the Transformers Catagory!

Add BBM and Epoch Japanese Grand Sumo trading cards, and more, under a new Sport Header for SUMO.  BBM and Epoch have been publishing licensed trading cards featuring wrestlers (Rikishi) from the top ranks of Grand Sumo for decades.  Cards feature the wresters in tournaments, everyday life, with their favorite things or activities.  There are historical cards, holo-cards, even autograph cards.  Memorabilia includes Tegata (handprints with signatures) and Banzuke (the schedule of bouts for sumo tournaments (basho), fans and other print published media.

The ability to specify a default folder so I don't have to use the Add with Details option every time.  Instead, I can add with one click, and it will automatically add the card into the folder I have specified as my default at that moment in time.

Instead of only track sold cards.  Add more support for trading cards.  So one item is removed from a collection and another item is added in it's place.