No spare key with used car

There's not much you can do, it's a used car and an issue with a missing second key would not be covered by the statutory warranty on used cars. It's one of those things that need to be sorted out before you accept the car. Once you've done that you've lost your bargaining power, but I would approach the dealer and try to come to an arrangement with them. I would think it would be well worth him spending $600 to have a satisfied customer.