It's usually good to use clear variable names that you can quickly understand ;)
For example "amount_needed" and "amount_available" instead of "n" and "a";


// check if OK with GetComponents
  var i, component_ID;
  while (component_ID = GetComponent(nil, i++, nil, potion_ID))
  {
    var amount_needed = GetComponent(component_ID, nil, nil, potion_ID);
    var amount_available = pClonk->ContentsCount(component_ID);
    if (amount_available < amount_needed)
    {
      Message("You need %d %s!", amount_needed, component_ID->GetName());
      Sound("Error");
      return;
    }
    // add ID X times to the list
    while (amount_needed--)
    PushBack(components, component_ID); // PushBack adds something to the end of the list
    // everything is ok!
  }

If you really want to use "pID" and "ID", you could replace all "component_ID" by pID and all "potion_ID" by ID in the code above. I hope I didn't get confused on the way and it should work now :)