As safe as it is to do anything that is against the original issuer's terms and conditions of use. You might skate - they might find you out (the voucher is then confiscated and you are asked to pay the full walk-up fare for the class you were booked in.) The seller / buyer might renege, and you would have no recourse. The odds? ¿Quien sabe? though we can always make wild guesses. Caveat whatever.