uses System.SysUtils,
  System.Generics.Collections;

type
  TCar = class
  end;

  TSeller = class
  public
    Cars: TList<TCar>;
    constructor Create;
    procedure Sell;
  end;

constructor TSeller.Create;
begin
  Cars := TList<TCar>.Create;
end;

procedure TSeller.Sell;
begin
  if Cars.Count = 0 then
    raise Exception.Create(
      'No cars for sale');
  //some implementation...
end;

var
  Seller: TSeller;
begin
  Seller := TSeller.Create;
  try
    Seller.Sell;
  except
    on E: Exception do
      WriteLn(E.Message);
      //E.Message is 'No cars for sale'
  end;
end.