That depends on the payment methods accepted by the ticket office where you buy it. But usually cash will be accepted.

I see you're from Argentina so you probably need thee Eurail pass. The more important question is then where you would like to buy it because it is only available from a limited number of ticket offices. At least the ticket offices listed here should sell it:
https://www.eurail.com/en/get-inspired/trains-europe/useful-train-information/eurail-aid-offices

Hola.

Pienso comprarlo en España, por lo que eh visto en Madrid o Barcelona lo venden.

Muchas gracias por la respuesta. 

Saludos.