The minute you contact them, they're going to think you're making money off their brand and get angry.
If they're big enough to have legal representation, you're going to find yourself at the receiving end of a Cease & Desist.
If they're big enough to have a following, but not big enough to have a lawyer - you're going to be on the receiving end of a message to their followers saying that you're selling unauthorized versions of their materials.
You would be better off registering their domains and before you try to sell anything on it, contact them and ask them if they would like to buy the domain or potentially partner on a merch deal where you take care of the product, site, hosting etc. And they promote it. In exchange, they get 70% of the revenue and you'll take 30%. That's a very attractive offer - and they might be willing since they don't have to do anything but promote the store.