Once Season Ticket Holders list their seats for sale on the viagogo website, they will receive anything from matchday face value to double the value of their matchday ticket, less a transaction fee.
There is no limit to the amount of games they can choose to sell – however there is no guarantee that their seat will be bought for any game.
Quite apart from the fact that viagogo are greatly disliked by many for their lack of business ethics, as exposed by Channel 4's dispatches "The Great Ticket Scandal" (Feb. 2012) I'm astounded that people can sell their seats for "up to twice the value of their matchday ticket" - although, of course, the supporter won't get anywhere near that sum once viagogo have taken their commission.
But under the The Criminal Justice and Public Order Act 1994 (section 166), The Violent Crime Reduction Act 2006 and The Ticket Touting (Designation of Football Matches) Order 2007, the unauthorised sale of a football ticket is a criminal offence, and if an ordinary supporter tried to sell a ticket for any price, let along "for up to twice the value of their matchday ticket" they'd be banged up - and many have.
No doubt the club will use the get-out that "it's not unauthorised as it's all being down through the club" and legally they're right. Morally, though, it sticks, even if it wasn't being done through a company quite as dislikeable and disreputable as viagogo.
Because why should a third-party company be able to make a profit out of enabling supporters to do something that legally they can't do themselves - and why can't the club themselves facilitate this in-house, thus avoiding an external company leeching off supporters?