I'd be willing to accept a vouch copy of this, too. If they were obtained legally, how can you offer them 50% off?
VOUCH COPY UPDATEI was sent a vouch copy of a 3 month Spotify code. Here are my account details before I applied the code, showing that it is quite obviously a free account (sensitive informatrion redacted):

I redeemed the code without a hitch, and I was told that I now have Spotify Premium. I was shown this page to prove that it worked:

As soona s I did this, Premium features became unlocked for my existing account that already had a premium subscription in the past, including offline playback. I got a receipt in my email and it seems so far that this is a legit product.
However, it usually happens that accounts that have gotten Premium subscription using illegitimate means (using a glitch/exploit in the Spotify system) will have their Premium subscription removed within a week or two. I can say this because it has happened to me before. As such, I will update this thread with more information in the coming days to let you all know if the code lasts or it doesn't. I would suggest that the seller should offer a at the very least a month's warranty on the codes so the buyer can be sure that if his premium is removed, he will not be at a total loss.
Thanks for your time.