You wouldn’t need to overthink this. I love the idea and they already have a system to deliver it to us if they wanted. Blizz simply sets another pvp vendor or vendors with the sets selling at 12 marks as always, but they would be locked until you qualify for purchase.
No Black market, making the purchase too limited and unfair.
No new currency for them to track and keep up with.
…and opens a door for them to introduce (test) pvp events to get people doing it. Like when the anniversary event happened, just for marks as always and the renown that unlocks the vendors.