If you are buying from a member on Bluemoon always checkout their history by clicking on their forum name. Someone who has history on here will most probably be legit. nw42 has left 4733 messages and joined in 2009 therefore I would think hes totally legit (especially if they have sold tickets previously). Someone who has only recently joined maybe not.
City have stopped allowing home printed tickets and are wanting everyone to join and have a membership account (you can get one for free but it will not allow you to purchase a ticket from City but it will allow others to transfer one to you).
Someone legit will ask you for your membership number and add you to their family and friends. They are then in a position to tranfer the ticket to you directly alternatively they may want to forward an email with the mobile ticket attached; in this case always add it your wallet immediately otherwise someone else can do so before you (just a warning). Hopefully happy days and you are coming to the match.