There are two conditions for you to be able to access this rather special seller: he will only appear on the roof of one of the houses, once the construction of Hateno Village is completed. For that, please refer to the guide below which details the associated quest thread.
Then, go on the roof of the northeast building of the village: Granté is waiting for you and offers you to buy, for 4000 rubies each, rare pieces of sets that can only be found by exploring Hyrule. Here are the sets of rare clothes concerned:
You are given the location and unlocking conditions of all clothing sets available in Zelda Breath of the Wild.
