Program: 1951
Project: 1951
Buggy source:
@Override public void run(){
  if (stop) {
    return;
  }
  if (npc == null || player == null || checkContainer(player.getHandle()) || checkContainer(npc.getHandle()) || player.getHandle().activeContainer != npc.getHandle().activeContainer || !player.isOnline()) {
    kill();
    return;
  }
  if (mode == Mode.STOCK) {
    return;
  }
  stop=true;
  int count=0;
  boolean found=false;
  for (  ItemStack i : npc.getInventory().getContents()) {
    if (!previousTraderInv.getItem(count).equals(i) && player.getHandle().inventory.j() == null) {
      restoreOldState();
      break;
    }
    if (!previousTraderInv.getItem(count).equals(i) && previousTraderInv.getItem(count).getTypeId() == mcPlayer.inventory.j().id) {
      found=true;
      handleTraderClick(count,npc.getInventory());
      break;
    }
    count+=1;
  }
  count=0;
  if (!found) {
    for (    ItemStack i : player.getInventory().getContents()) {
      if (!previousPlayerInv.getItem(count).equals(i) && player.getHandle().inventory.j() == null) {
        restoreOldState();
        break;
      }
      if (!previousPlayerInv.getItem(count).equals(i) && previousPlayerInv.getItem(count).getTypeId() == mcPlayer.inventory.j().id) {
        handlePlayerClick(count,player.getInventory());
        break;
      }
      count+=1;
    }
  }
  clonePlayerInventory(npc.getInventory(),this.previousTraderInv);
  clonePlayerInventory(player.getInventory(),this.previousPlayerInv);
  mcPlayer.inventory.b((net.minecraft.server.ItemStack)null);
  Packet103SetSlot packet=new Packet103SetSlot(-1,-1,null);
  mcPlayer.netServerHandler.sendPacket(packet);
  stop=false;
}

Fixed version (ground truth):
@Override public void run(){
  if (stop) {
    return;
  }
  if (npc == null || player == null || checkContainer(player.getHandle()) || !player.isOnline()) {
    kill();
    return;
  }
  if (mode == Mode.STOCK) {
    return;
  }
  stop=true;
  int count=0;
  boolean found=false;
  for (  ItemStack i : npc.getInventory().getContents()) {
    if (!previousTraderInv.getItem(count).equals(i) && player.getHandle().inventory.j() == null) {
      restoreOldState();
      break;
    }
    if (!previousTraderInv.getItem(count).equals(i) && previousTraderInv.getItem(count).getTypeId() == mcPlayer.inventory.j().id) {
      found=true;
      handleTraderClick(count,npc.getInventory());
      break;
    }
    count+=1;
  }
  count=0;
  if (!found) {
    for (    ItemStack i : player.getInventory().getContents()) {
      if (!previousPlayerInv.getItem(count).equals(i) && player.getHandle().inventory.j() == null) {
        restoreOldState();
        break;
      }
      if (!previousPlayerInv.getItem(count).equals(i) && previousPlayerInv.getItem(count).getTypeId() == mcPlayer.inventory.j().id) {
        handlePlayerClick(count,player.getInventory());
        break;
      }
      count+=1;
    }
  }
  clonePlayerInventory(npc.getInventory(),this.previousTraderInv);
  clonePlayerInventory(player.getInventory(),this.previousPlayerInv);
  mcPlayer.inventory.b((net.minecraft.server.ItemStack)null);
  Packet103SetSlot packet=new Packet103SetSlot(-1,-1,null);
  mcPlayer.netServerHandler.sendPacket(packet);
  stop=false;
}