Channel4 did a good documentary on Viagogo and they found what we are seeing here. Yes, it's a ticket exchange for fans but Viagogo also get a batch of tickets off promoters (City in this case) to sell at inflated prices. That's why the best seats at arenas never ever go on public sale.
I could live with supporters coming late to the party, buying the odd ticket and being charged a premium. What's not nice to see is that people have to be City members to get these tickets. Thats just milking us for profit.