I've sold two lots of tickets this season. I'm a season ticket holder and my seats are linked to viagogo. You enter your reference number when you register. When I sell tickets on viagogo, my season ticket card is cancelled for that game. I suppose buyers are either posted tickets or can print off their own.
I've done quite well really. I sold two tickets for Liverpool for £54 each (netted about £95 after seller fees) and also 2 x Everton for £46 each (netted £81). I feel like a right ticket tout.
I think it's a racket, but I suppose it's worked for me.