January is traditionally a bit slower for most, as people have spent a lot on Christmas, and often on Boxing day / new year sales as well, then families get ready for back to school at the end of January, so a lot of stores will feel the pinch as cash gets saved after the big spending or diverted to those things.
Things can also be a bit slow if you go on holidays, if you hide your listings during that time, because then for x amount of time your listings get pretty much no activity so they can suffer a little in eBay rankings.
Re: store categories, you don't have to create your own, you can also choose to just display eBay's categories in your store, but creating your own does give you the opportunity to let buyers refine by different things, or create interest-drawing categories (eg "Clearance", or "New Arrivals", new arrivals needs to be maintained consistently, though, putting new items in, removing older items. Refining by size is popular, and while you can do that in search results on eBay, clothes aren't categorised by eBay by size).
A lot of your photos look a little on the warm side so when you edit them, you might find they look a lot better if you turn that down slightly - if you'd like to make them pop a bit more, check out Fotofuze. They have a free version if you don't mind the maximum size of your image being 700 x 700, (USD$5 per month if you want larger images), and it's all done automatically after you highlight the item in your photograph. I took the liberty of grabbing one of your gallery images at random to put it through the image tool, which I've put in the spoiler below. I also slightly adjusted the warmth and made it a little brighter - Fotofuze can be a bit funny with whites and shadows, so it basically 'fuzed' the white of the dress as well, and I had to touch it up manually to remove some dark shadows as well as the coathanger, but even if not perfect, it's an example of how much difference a bright image with a clean background can make).
