I have not long started with Spreadshirt & have been promoting my Tees etc. on Facebook through my pages & sponsored ads. I have around 450 likes on one of the posts and about 250 on the other and not one sale 
My prices for my tees are pretty standard (£18.99) or there about.
With a good reaction to these posts I would expect to get at least one sale.
Any advice on where I could be going wrong?
I think the issue is that fundamentally, people don’t go on Facebook to buy stuff. So, you might get great brand recognition and engagement, but your audience might not convert right away.
They might like your page and remember your t-shirts a few weeks from now when they feel like they’re ready to commit to the purchase.
I personally have very similar experiences and in my opinion, you should focus on brand building on Facebook by sharing & producing relevant and engaging content. If you share your shop link, I can give you a few ideas 
Once you have built yourself an audience, it’s easier to convert them to happy, paying customers
You need to be able to be on the mind of the people outside Facebook - because that’s where the shopping happens!
Hi
With a vast experience in marketing and graphic design a good advice would be to take a online course with Lynda (lynda.com) or Udemy (udemy.com). I use both of them to stay on top and to learn new subjects. I can really recommend Digital Marketing Masterclass on Udemy and the Become a Social Media Marketer on Lynda.
Hope this is some kind of help.
Regards from Sweden
Oh yes definitely! A vote for Lynda & Udemy from here, too!
You can learn so much cool stuff there for peanuts.
@User_EU-bb69bea6-9b20-4080-8379-82cc4f6e9cd3
Hejssan, neighbor! Nice to see some Nordics here 