Une autre possibilité : expliquer la situation en anglais au vendeur, et voir s'il accepte d'annuler la vente. S'il refuse, il faudra le payer.
Voilà ce que vous pourriez lui écrire :
I am extremely sorry, but I have put an auction on your gameboy (my son wanted one) without realizing that it was an english version. Nobody in my family speaks english so we cannot use the gameboy (I have asked somebody to translate this message into english). Would you accept to cancel the purchase, and accept my apologies ?
I am looking forward to your answering.
Best regards, (pseudo)