Hey, Max,
It's a simple thing, if you boil the issue down to brass tacks.
What are the other playing card dealers doing in the marketplace? For example, Ellusionist has some great decks, but for the most part, they're offering just their own decks and that's it - same for Theory11. Look at these sites, and look especially at sites that aren't magic-centric but instead are card-centric, like playingcards.net, playingcarddecks.com, collectableplayingcards.com, artofplay.com and so on - I'm sure there are others I'm missing. What are they doing? What do you think of their sites? What are they getting right, and what are they getting wrong? Look at where you can either outperform them at what they're doing, or look at what niche they're not covering very well and see if you can cover it better.
Try placing a sample order from each site. What's the user experience like for you? Are there ways you can do it better?
Is their selection top notch? Are there some gaps that you think you can cover that they aren't?
Is their shipping costly? Are there ways you can beat them at that?
I remember, years ago, shopping for my sensei for a new hakama to present to him as a birthday gift. I actually found a martial arts specialty store that was located in Japan, but they created their website specifically to cater only to customers outside of Japan. It was all in English (available in other European languages as well), and it was pretty well-translated, with few poorly formed or unrecognizable phrases or sentences that didn't make any sense. They even managed to make their shipping costs reasonable, to the point that I wouldn't be able to find a better deal in a US-based store. This company found a way to compete in a highly competitive marketplace by finding an underserved niche and catering to it exclusively - tear a page from their textbook and do the same!
Good luck to you.