Villagers will not buy items for the discounted rates when Hero of the Village is active if you do not have enough of the item requirement prior to the HotV discount. For example, if the prior trade was 7 Melon -> 1 Emerald, and the HotV trade was 3 Melon -> 1 Emerald, the Villager would not accept the 3 Melon -> 1 Emerald trade unless I had 7 Melon. If I attempt this, it seems like the trade goes through, but it is only a visual glitch, and looking at my inventory immediately after the trade shows that it hasn't gone through. This is occurring in my Minecraft Realm, but I haven't tested this on a Multiplayer server or Singleplayer world.