I agree security has to be high however ultimately The starting point of any ticket is through the club selling it. You only have to look online to see the numerous ticketing websites offering tickets at inflated prices, Watford last season, I found a website offering 10 together In the away end, you get touts outside the grounds, you have the numerous corporates/joes potentially passing tickets on, mates who will buy and pass to a mate, but thats all mainly for away games. Ultimately if you look at the ticket exchange site in question of which 99.9% of transactions relate to home tickets the aim is just to sell empty seats yo help fill the ground and potentially give single parents who are on low incomes the chance to take her young son to a game in the hope he will grow up to be a city fan and to ensure that city fans have an option where they can get some money back now for a game they cant make rather than wait upto 12 months for a credit. Ultimately there is no way City can ensure that the ticket they sold to mr x is used by Mr x all they can do is ensure that whoever turns up that day is vetted before entry to the stadium and as in all situations ultimately if any issues the original purchaser is the one held accountable.
Whatever people's views are on the site being talked about there is zero evidence of any ticket being used by an away supporter and in terms of any potential terrorist threat I'm sure any potential bomber would easily be able to purchase a ticket via the club if they so wished as doubt every purchaser is ran through a terror watch list.