There are too many things to list individually but as the first point sof call:
That logo needs to change. In fact the header as a whole looks squashed
As people have said it lacks basic design principal. There is too much going on and the layout is hard to navigate
It just does not look premium
The colours could do with rethinking – its too baby and cold
Generally the typography needs tightening in terms of fonts, typeface and spacing
As everyone said take a look at the big sellers and the attention to detail and tidyness in them. The big thing here for me is there are so many “bits” and features but not enough focus on user experience and polished design for the site.
You have the code there so its much more aesthetic more than anything else.