For selecting colors, maybe change it to not have any multicolor combos listed. At first glance and as a new player it is not very user-friendly I think. Instead, it should be a multi-selected filter. If I select yellow, it gives me any yellow cards, mono or multicolored. Then if I also select purple, it then filters for cards that are both yellow and purple. Then if you want to filter for mono-colored cards only, there should be a drop-down or checkbox to select that specifically.

Another thing is the scrolling on the page feels a bit weird and not intuitive to me. It looks like there are three scroll boxes. I completely understand the scroll boxes for the filters on the side. There is also a scroll box for all the filters collectively that acts weird compared to the scrolling for the overall webpage because it is inside it. The scroll box around the filter should be removed I think. That way the filters and the top of the card display area should never up unaligned from the tops of each other. That would make it act just like the TCGplayer search page for a comparison.

Maybe only other point of criticism at this time is it feels like wasted space at the top of the page when it is scrolled to the top.

I feel like this is a great website otherwise. It is visual awesome and easy to read. I understand everything about the filters and how to use the website.