It could be as simple as the people that are visiting your site through those places aren't the people who are interested in buying your t-shirts.
It could be that people aren't interested in buying a design with your website address printed on the front of the t-shirts. That's usually something that *really* popular blogs can pull off. On second look, it looks like the actual t-shirt doesn't have your website printed on the front, but your watermark on the main product image makes it look like it. I'd change that immediately to show an actual photo so that people know your website address isn't plastered on the front of the t-shirt.
It could be that the designs aren't compelling enough to make people want to pull out their wallet and buy.
It could be that you don't have their size (statistics show over 10% of shoppers where XXL or larger)
It could be that people don't have money this week.
I've read some statistics that most people need to view an ad at least 7 times before taking an action. A week really isn't that long.
How many people have actually visited the shop? Conversion rates for browsers to buyers is as low as 1%-2%. If your visitors are untargeted people no interested in buying, that number could be even lower.
You may have misjudged the interest in people wanting to buy t-shirts with those designs.
A Facebook like for a picture posted is not the same as actual interest in spending money to buy a t-shirt. 4 Facebook likes really isn't a good gauge that a t-shirt is worth printing. If you are getting a LOT (10+) of people commenting saying "where can I buy this t-shirt?" and "can I preorder this t-shirt", THEN you've got a better barometer of t-shirt interest.
I'd suggest following up with a comment on this Facebook post from a month ago to let those people know who have commented and liked that post that the t-shirts are now printed and available for sale:
https://www.facebook.com/photo.php?....281390431877125.86567.167809746568528&type=1
People may be coming to your site to read your blog posts, but not really interested in buying t-shirts. You have to really put yourself out there that you are now selling t-shirts. A banner off to the side and one blog post might not be enough.
There could be a lot of reasons why you haven't sold a t-shirt in a week.
The real question is, what other marketing plans did you have in mind to help get those t-shirts sold. It's not always as easy as a cool t-shirt design. The right advertising and marketing can be the difference between a cool t-shirt sitting on the shelf of selling like tasty pancakes.
Does Hipp have fans? Does his fanbase know that his artwork is now available for purchase on your site?
You may want to clean up your bigcartel images so that the actual photographs of the t-shirts are more prominent.
Lose the watermarks (they probably aren't necessary for a photo and can lead to some confusion).
Make the product photos bigger on the homepage and product pages.
Link the banner on your homepage to the actual page on your site with that t-shirt design for sale instead of making the user click from the shop homepage to get there. You enticed them with a nice photo of one particular design, don't send them to the main page and make them click again...especially when you only have 2 t-shirts to choose from. Better to increase the chances of closing that sale of the t-shirt design you featured.
With only 2 t-shirt designs, you could create a banner for each design and rotate it in that same spot on your homepage and link it directly to the add to cart page for that design.
Add information about the t-shirt quality (are they soft, what brand?), printing, shipping costs, sizing to the add to cart page so people don't have to click around to find it (or be surprised by shipping costs).
What is your return policy? You have to earn the trust of potential shoppers to get them to spend their hard earned money on your t-shirts. Who are you, how do they contact you? Do you have a phone number? How do they know you're not a fly by night operation that will take their money? Sounds like a simple question, but indicators like phone numbers, addresses for returns, return policies, etc can help act as trust signals to potential shoppers.
Why would someone want to buy your t-shirt? How does it benefit them? That could be added to the shirt description on the "add to cart" page. Will it make them cool? Is it limited edition? Will it help support their favorite blog? Don't make them click away from the "add to cart" page to get all the information they'd need to be compelled to buy.
Hope this helps some