Eddie is right. I don't understand why they need the Spanish ID card number (DNI #) to book a ticket.
I guess that, providing they can charge you the tickets on your credit card, they won't care whether you give them a DNI or your passport number.
Be aware that, sometimes, if you book things over the phone or the Internet you may be there 1 or 2 hours in advance to collect the tickets (I think this is what they do at Auto-Res)
Regards,
Antonio
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