The merchant will gray out all the cards that you can't use due to not having the right heroes to go with them, but the merchant also has a limited "stock," so to speak. It's sort of a strange system, but the merchant can actually run out of cards. It starts off with a supply of all of them, and as people buy/sell its inventory changes. But because more people buy than sell, a lot of cards eventually sell out, so every few months they reset the merchant, and we call this the merchant getting "restocked." So possibly the merchant had sold out of a lot of cards, and now it's restocked, so you're seeing new ones that weren't there before.
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