I could be wrong, but I though everyone who bought early Primes had the buyback option, and everyone who exercised the buyback option also had the "reverse" sale button.
I spent enormous amounts of time clicking through ZenCloud at the time when all this was going on and I could never get the buyback to work for me. I think it only worked if there was someone willing to buy your primes at full retail? I don't remember now, it was so convoluted.
That was the original plan, but for 2-3 weeks that did not work or only worked for a few lucky folks. One of the resellers (Volker I think) made a public offer to buy back any Prime at cost - payours, then shortly thereafter GAW ended up offering the $45 - payouts option which is what I took up. I can't find the link to the HT post, but it was in the Announcements when GAW made this decision.
The formula was very strange. On several of my primes the buyback resulted in very small or no payment, and in others I seemed to get the full value. I never figured out how they calculated the $45 - payouts but I took what I could get.