An exercise in self-justification, I thought about how these might have been acquired. That the seller will not tell us - ostensibly due to protecting his legitimate business interests and keeping away competition - is in itself suspicious, but I set that aside for now.
1) A £100 iTunes voucher cannot have a cost of £100, only a value. To create the voucher takes less than pennies, the songs do not have a cost of 79p. There is surely room for wiggle.
2) This kind of item is such a PR toy - they probably get handed out left right and center to all sorts of people and companies. So if they are indeed stolen, then it's possible it was just lifted from some Soho storage cupboard where they were sitting for months. White collar crime is fine!
3) I would really quite like to go to prison anyway.