ProtocolManager protocolManager = ProtocolLibrary.getProtocolManager();

PacketAdapter.AdapterParameteters params = PacketAdapter.params().plugin(this)
    .listenerPriority(ListenerPriority.NORMAL)
    .types(PacketType.Play.Server.OPEN_WINDOW_MERCHANT);

// Modify merchant recipes (normal priority)
protocolManager.addPacketListener(new PacketAdapter(params) {
    @Override
    public void onPacketSending(PacketEvent event) {
        PacketContainer packet = event.getPacket();

        // Set result of each trade to the coal item
        List<MerchantRecipe> merchantRecipes = packet.getMerchantRecipeLists().read(0);
        for (MerchantRecipe merchantRecipe: merchantRecipes) {
            merchantRecipe.getResult().setType(Material.COAL);
        }

        // Set merchant level to 3 and experience to 100
        packet.getIntegers().write(1, 3);
        packet.getIntegers().write(2, 100);
       
        event.setPacket(packet);
    }
});

// Log the changed packet (highest priority)
protocolManager.addPacketListener(new PacketAdapter(params.listenerPriority(ListenerPriority.HIGHEST)) {
    @Override
    public void onPacketSending(PacketEvent event) {
        PacketContainer packet = event.getPacket();

        int windowId = packet.getIntegers().read(0);
        int villagerLevel = packet.getIntegers().read(1);
        int villagerExperience = packet.getIntegers().read(2);
        boolean isRegularVillager = packet.getBooleans().read(0);
        boolean canRestock = packet.getBooleans().read(1);
        List<MerchantRecipe> merchantRecipes = packet.getMerchantRecipeLists().read(0);

        String tradeList = merchantRecipes.stream()
            .map((MerchantRecipe recipe) -> recipe.getIngredients().stream()
                .map((ItemStack itemStack) -> itemStack.getType().toString().toLowerCase())
                .collect(Collectors.joining(" + ")) + " => " + recipe.getResult().getType().toString().toLowerCase())
            .collect(Collectors.joining(", "));

        event.getPlayer().sendMessage("TradeList(Window=" + windowId +
            ", Lvl=" + villagerLevel + ", Exp=" + villagerExperience + ", CanRestock=" + canRestock +
            ", IsRegular=" + isRegularVillager + ", Trades=[" + tradeList + "]");
    }
});