Same issue here, but thankfully there's a workaround
What seems to be happening is about 15-30 seconds after loading the page, it "resets" and clears your choices. If you try to move forward before it "resets", it will say you haven't chosen anthing. So, to get around it, this is what I do:
-Load the page and immediately check any of the categories
-Wait 15-30 seconds until the check dissapears
-Choose the actual category I want and proceed
This bug has been around since at least May, possibly much longer. (That's roughly when I started selling)