Program: 2081
Project: 2081
Buggy source:
private void handlePlayerItemClicked(int slot){
  player.getInventory().setItem(slot,previousPlayerInv.getItem(slot));
  ItemStack i=player.getInventory().getItem(slot);
  if (!npc.getTraderNPC().isSellable(i.getTypeId())) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Sellable sellable=npc.getTraderNPC().getSellable(i.getTypeId());
  int amount=player.getInventory().getItem(slot).getAmount();
  if (amount - sellable.getSelling().getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(sellable.getPrice(),npc)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  ItemStack transfer=sellable.getSelling();
  HashMap<Integer,ItemStack> unsold=npc.getBukkitEntity().getInventory().addItem(transfer);
  if (unsold.size() >= 1) {
    npc.getBukkitEntity().getInventory().setContents(previousNPCInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(sellable.getPrice(),npc);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.GREEN)+ "String_Node_Str"+ StringUtils.yellowify(MessageUtils.getPriceMessage(sellable.getPrice()),ChatColor.GREEN)+ "String_Node_Str");
  npc.getBukkitEntity().getInventory().setContents(sortInventory(npc.getBukkitEntity().getInventory()));
}

Fixed version (ground truth):
private void handlePlayerItemClicked(int slot){
  log("String_Node_Str" + (previousPlayerInv.getItem(slot) == null || previousPlayerInv.getItem(slot).getType() == Material.AIR));
  player.getInventory().setItem(slot,previousPlayerInv.getItem(slot));
  ItemStack i=player.getInventory().getItem(slot);
  log("String_Node_Str" + (i == null || i.getType() == Material.AIR));
  if (!npc.getTraderNPC().isSellable(i.getTypeId())) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Sellable sellable=npc.getTraderNPC().getSellable(i.getTypeId());
  int amount=player.getInventory().getItem(slot).getAmount();
  if (amount - sellable.getSelling().getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(sellable.getPrice(),npc)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  ItemStack transfer=sellable.getSelling();
  HashMap<Integer,ItemStack> unsold=npc.getBukkitEntity().getInventory().addItem(transfer);
  if (unsold.size() >= 1) {
    npc.getBukkitEntity().getInventory().setContents(previousNPCInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(sellable.getPrice(),npc);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(sellable.getSelling().getAmount() + "String_Node_Str" + sellable.getSelling().getType().name(),ChatColor.GREEN)+ "String_Node_Str"+ StringUtils.yellowify(MessageUtils.getPriceMessage(sellable.getPrice()),ChatColor.GREEN)+ "String_Node_Str");
  npc.getBukkitEntity().getInventory().setContents(sortInventory(npc.getBukkitEntity().getInventory()));
}