Not sure whether its illegal to resell a ticket. When you go to Wimbledon I believe you have to produce id to show that the ticket belongs to you. Technically I believe the seat belongs to whoever has the ticket at the time. 'Not for resale' is very debatable in law. Perhaps you legal eagles have an opinion on that. Having said that, we live in a free market economy. Basically what that means is that you can sell anything for what anyone is willing to pay, whether its houses, cars, football tickets or whatever. Why should concert tickets be any different?
ps I have heard that some ticket agencies sell tickets that that they dont even possess at the time. As bad as touting if you ask me.
pps It seems the normal now, if you can get through, to buy your allocation of four tickets which you can then sell to anyone you like. Ticket agencies employ students to do it.