List more items. Many sellers indicate that once they had 80-100 items in their shop, it made a big difference. I have another shop (hand painted ornaments) and once I had ALOT listed items, sales and views increased significantly.
Look at other art sellers - especially successful ones. What are they doing? I especially agree with Jessica, above, who indicates that you need more info in your descriptions. Of course you do! Tell your customers more...give them a reason to want to buy from you. Use all of your picture options for your listing. Show your art framed. Show your pictures hanging on a wall. Show your art matted in different ways. Your tags also need work. You use generic tags (art, modern art, print, abstract art, etc) - how about tags that are more specific to each drawing?
While your artwork is very good, it appeals to a very specific audience. This is fine if you have sufficient customers that you reach (and that want to buy) in this target arena. But, if you are looking to broaden your reach you may want to consider something with more mass appeal.
I don't spend $ on advertising and would not recommend it. Take care of all the other things first.
You are off to a good start! Good luck.