Stitches on my sneaker were coming out in a day of wearing them

I bought the sneaker at cross trainer and stitches were coming out after a day of wearing the sneaker. I went back to the store they told me they can't refund because it is a used item. How can expensive sneaker like that 😫 the stitch come out within a week? They took my number and they told me they will communicate with head office and revert till today. I'm very disappointed,I WILL NEVER EVER STOP AT ANY CROSS TRAINER,even people close to me I'll advise them not to do so.