After you have completed the Tarrey Town questline at least up to the point when Hudson asks you to invite Bolson and Karson to the wedding, a new merchant will be availiable. Granté can be found on the balcony of the house with the donkey in the front yard.
This merchant sells some "unique" armor that you previously owned, but when and how much depends on the type of armor:
Armor that can be bought elsewhere: He will never sell that. If you sold things like the Snowquill armor or the Flamebreaker set, then you will have to buy those in the original shop again.
Unique Base Game armor: If you found at least one part of a set that is NOT sold in any regular shops, then Granté will carry the whole set. If you, for example, found only the Rubber Helmet but not the chestpiece and trousers yet, you can simply buy them from Granté. (proof - zero Rubber Tights in inventory, still buyable)
DLC armor: He will sell only the individual pieces you have previously found, unlike the other sets. If you, for example, found the Phantom Ganon chestpiece and trousers but not yet the helmet, Granté will ONLY sell the chestpiece and trousers. If you want the third part of the set, you need to complete the respective scavenger hunt first. (proof - Granté does not sell the helmet)
Armor that can not be sold: You can not sell the Zora Armor, Armor of the Wild, Thunder Helm, and Champion's Tunic. Despite these sets being "unique", Granté doesn't sell them. Why? Because this merchant was added to BotW as a means of recovering armor that the player might have accidentally sold, and since you can not get rid of these items in the first place, there is no need for a recovery service. You CAN lose two of these due to a glitch (try to obtain them while your armor inventory is already full), but Nintendo obviously didn't expect that to happen.
Amiibo armor: Despite these being "unique" in the sense that no in-game shop sells them, Granté won't sell them either. If you accidentally sold a piece of amiibo armor and you want it back, you have to keep scanning the amiibo and hope that RNG blesses you with the desired outcome.
All armor will be at base level regardless of whether or not you had upgraded the original ones already. They will also be un-dyed even if you dyed the original sets. You can buy duplicates if you want to, but you have to wait for him to restock as he only carries one item of each type per restock cycle.
Another thing that Granté may sell, is the Hylian Shield. This, too, has some specific conditions:
You need to have obtained the original Hylian Shield first, or at least opened the chest containing it (it doesn't matter whether you take it OUT of the chest as long as you opened it. If your shield inventory is already full, Link will close the chest but the game will set the flag that you obtained the shield)
No Hylian Shield currently exists. You have to break or despawn the shield you currently own in order to make him stock one replacement, and the same applies to any duplicates you might have cloned with glitches. If you have 3 Hylian Shields in your inventory and 2 more on the weapon mounts in your house, you will have to destroy ALL five of them in order to be able to buy one replacement.
....you can infinitely repeat this tho. Buy a replacement - break the replacement - buy another replacement - break that one as well - buy a third replacement ... he never stops restocking them, so you can never permanently run out of Hylian Shields.
There is one downside to this: The Hylian Shield from the chest is able to get bonuses / modifiers. This happens very very VERY late and requires a whole lot of work to make it happen, and it applies ONLY to the shield from the chest. If you buy a replacement later, it won't have a bonus even if the original shield had one.
Granté does not sell any other "unique" equipment, so you can not use him to get more Kite Shields or reforge the Scimitar of the Seven and the like. It's only the Hylian Shield, and nothing else.