Are you willing to provide a vouch copy? I haven't seen these codes up for sale on the forum before, how can you manage to sell them so cheap? Are you able to say where you obtained these codes? Surely not officially/legimately, right?
Well the 12 month codes are generated by Spotify intended for promotional deals (when you get free spotify with for instance your internet provider, you get these codes). The 24 month codes are far less known because they are only intended for Spotify employees and their family/friends. A few of these codes are generated for spotify employees every month. Now most employees don't use the codes because everybody they know already has premium, so they are unused. Now being a technical employee at spotify, I managed to get access to the log file of generated codes. I took a copy of the log and reported the problem to my boss. Now I can sell the codes in the log.