Program: 2060
Project: 2060
Buggy source:
private void handlePlayerItemClicked(int slot,PlayerInventory playerInv){
  playerInv.setItem(slot,previousPlayerInv.getItem(slot));
  ItemStack i=playerInv.getItem(slot);
  if (!npc.getTraderNPC().isStocked(i.getTypeId(),i.getData(),true)) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Stockable stockable=npc.getTraderNPC().getStockable(i.getTypeId(),true);
  ItemStack selling=stockable.getStocking();
  if (previousPlayerClickedSlot != slot) {
    previousPlayerClickedSlot=slot;
    player.sendMessage(ChatColor.AQUA + "String_Node_Str" + MessageUtils.getStockableMessage(stockable,ChatColor.AQUA)+ "String_Node_Str");
    return;
  }
  previousPlayerClickedSlot=slot;
  int amount=playerInv.getItem(slot).getAmount();
  if (amount - selling.getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(new Payment(stockable.getPrice()),npc)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  HashMap<Integer,ItemStack> unsold=npc.getInventory().addItem(selling);
  if (unsold.size() >= 1) {
    npc.getInventory().setContents(previousNPCInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(new Payment(stockable.getPrice()),npc);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str");
}

Fixed version (ground truth):
private void handlePlayerItemClicked(int slot,PlayerInventory playerInv){
  playerInv.setItem(slot,previousPlayerInv.getItem(slot));
  ItemStack i=playerInv.getItem(slot);
  if (!npc.getTraderNPC().isStocked(i.getTypeId(),i.getData(),true)) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Stockable stockable=npc.getTraderNPC().getStockable(i.getTypeId(),true);
  ItemStack selling=stockable.getStocking();
  if (previousPlayerClickedSlot != slot) {
    previousPlayerClickedSlot=slot;
    player.sendMessage(ChatColor.AQUA + "String_Node_Str" + MessageUtils.getStockableMessage(stockable,ChatColor.AQUA)+ "String_Node_Str");
    return;
  }
  previousPlayerClickedSlot=slot;
  int amount=playerInv.getItem(slot).getAmount();
  if (amount - selling.getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(new Payment(stockable.getPrice()),npc)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  HashMap<Integer,ItemStack> unsold=npc.getInventory().addItem(selling);
  if (unsold.size() >= 1) {
    npc.getInventory().setContents(previousNPCInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(selling.getAmount() + "String_Node_Str" + selling.getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(new Payment(stockable.getPrice()),npc);
  EconomyHandler.pay(new Payment(stockable.getStocking(),stockable.getPrice().isiConomy()),player);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str");
}