Saying that they're not fit for purpose? Tricky one and almost certainly not worth the cost of a small claim!!!
Did you try the shoes in the shop?
If not, then I'd doubt you can complain that they're not right for you.
Also, in my experience Easy Runner say that you should try new shoes on the treadmill so that if they don't work for you they will still look pretty 'box fresh'.
Against that, think how you'd feel if you were sold a pair of shoes that had 30/50 miles on the clock - peeved, I should think.
There's no 'right' answer to this one, imho.