I am pretty sure this is not allowed. Due to it is related to buying/selling/trading bot authorization codes. You should edit out post, before you get infracted.
Idk... TECHNICALLY he is trying to sell his rsbuddy account and not his auth codes themselves so this MIGHT be like walking on the line of legal on sythe. I saw a guy on ebay a LONG while back selling a sock for $50 and if you bought the sock you would get a "free" runescape account with it and he got a away with it lol... idk, just a thought