I have used them, yes. Zazzle is a huge US based site which markets thousands of products for independent designers and artists.
As far as I am aware, Warner Bros Entertainment Inc own the copyright to Harry Potter merchandise. J K Rowling and her associates are famously litigious so one would hope that Zazzle have obtained rights to sell this stuff but you may wish to double check it's legitimate. It's easy to print an image on a T-shirt and fake a copyright label.
Often Zazzle goods are made to order so be prepared for a delay in delivery and be aware of customs duties, VAT and handling charges for expensive items.
I don't buy a lot off the internet. I would like to buy a child's t-shirt from this site. It says the t-shirt's are official J K Rowling products. Has anyone bought anything from here? It looks like it's a site in the USA.