Shop NPC

    private void shopSetup() {
        // Create a default Steve skin
        Skin shopSkin = new Skin();
        shopSkin.setSkinId("shopkeeper");
        shopSkin.setSkinData(new byte[8192]); // default blank skin data
        shopSkin.setCapeData(new byte[0]);
        shopSkin.setGeometryName("geometry.humanoid.custom");
        shopSkin.setGeometryData(Arrays.toString(new byte[0]));

        // Spawn a test NPC at your spawn
        Vector3 testPos = new Vector3(65.5, 6, -45); // slightly above ground
        CallbackNPC testNPC = new CallbackNPC(level, testPos, "TestShop", shopSkin, p -> {});
        testNPC.spawnToAll();

        for (Team t : teams.values()) {
            NPCShop npcShop = new NPCShop(t.getColorCode() + t.getColor() + " Team Shop");
            npcShop.addItem(Item.get(Item.STONE_SWORD, 0, 1), 10, NPCShop.Currency.IRON);
            npcShop.addItem(Item.get(Item.IRON_INGOT, 0, 16), 5, NPCShop.Currency.GOLD);

            Vector3 shopPos = t.getSpawn().add(3, 0, 0);

            CallbackNPC shopNPC = new CallbackNPC(level, shopPos, t.getColorCode() + t.getColor() + " Team Shop", shopSkin, player -> {
                SimpleForm form = new SimpleForm(npcShop.getName(), "Select an item to buy:");

                npcShop.getItems().forEach((itemKey, shopItem) -> form.addButton(shopItem.item().getName() +
                        " §7- Price: " + shopItem.price() + " " + shopItem.currency().name()));

                form.send(player);
            });

            npcs.add(shopNPC); // store strong reference
            level.getChunk(shopPos.getFloorX() >> 4, shopPos.getFloorZ() >> 4, true); // force chunk load
            shopNPC.spawnToAll();
        }
    }

public abstract class AbstractNPC extends EntityHuman {

    protected final String npcName;

    public AbstractNPC(@NotNull Level level, @NotNull Vector3 pos, @NotNull String name, @NotNull Skin skin) {
        super(level.getChunk((int) pos.x >> 4, (int) pos.z >> 4), new CompoundTag()
                .putList("Pos", new ListTag<DoubleTag>()
                        .add(new DoubleTag(pos.x))
                        .add(new DoubleTag(pos.y))
                        .add(new DoubleTag(pos.z)))
                .putList("Motion", new ListTag<DoubleTag>()
                        .add(new DoubleTag(0.0))
                        .add(new DoubleTag(0.0))
                        .add(new DoubleTag(0.0)))
                .putList("Rotation", new ListTag<FloatTag>()
                        .add(new FloatTag(0f))
                        .add(new FloatTag(0f)))
                .putCompound("Skin", new CompoundTag()) // empty skin
                .putString("NameTag", name)
        );
        this.setSkin(skin); // must be called BEFORE spawn
        this.npcName = name;
        this.setNameTag(name);
        this.setNameTagVisible(true);
        this.setNameTagAlwaysVisible(true);
        this.setPosition(pos);
    }

    /**
     * Called when a player interacts with this NPC
     * Implement in subclasses to open shop/upgrade menus
     */
    public abstract void handleInteraction(@NotNull Player player);

    /**
     * Spawns this NPC to all players in the level
     */
    public void spawnToAll() {
        level.loadChunk(getPosition().getFloorX() >> 4, getPosition().getFloorZ() >> 4);
        for (Player p : level.getPlayers().values()) {
            this.spawnTo(p);
        }
    }
}

public class CallbackNPC extends AbstractNPC {
    private final Consumer<Player> onInteract;

    public CallbackNPC(Level level, Vector3 pos, String name, Skin skin, Consumer<Player> onInteract) {
        super(level, pos, name, skin);
        this.onInteract = onInteract;
    }

    @Override
    public void handleInteraction(@NotNull Player player) {
        onInteract.accept(player);
    }
}