It's easy to bash the Marlins for a variety of things, especially since they're ever so willing to provide the material with which to rip them with. Their after-the-fact selling of tickets to Henderson Alvarez's no-hitter, though, shouldn't be part of that bunch. Yes, it seems like something that's a bit scummy, but that's just the Marlins flavor in your mouth: there have been, and likely will continue to be, teams that sell tickets to events that already happened, just like this.
This was actually something of a mini controversy just last summer, when Johan Santana threw the first no-hitter in Mets history and New York sold the unsold tickets for face value as well as reproductions of each and every ticket for a base price. New York Magazine ran a retrospective in response to the outcry detailing recent times where teams have done a similar thing in selling unsold tickets after the event had already concluded.
In 2009, the White Sox charged face value for all of the unsold tickets from Mark Buehrle's perfect game against the Rays. In 2010, the Marlins sold tickets to Roy Halladay's no-no, when the Phillies starter came to Miami and dismantled the then-Florida Marlins. The scummy (read: Marlins-esque) part was that they planned to count these tickets sold after-the-fact towards the game's attendance. That, and the face value they sold their tickets for ($12 to $300) far surpassed that of Chicago's ($23 and $57). See, I told you they give you plenty to work with without having to make up reasons to be mad.
The Rays went about things a bit differently in 2010, as they gave away unsold tickets to Matt Garza's no-hitter as part of a ticket package that went towards a Joe Maddon fundraiser -- it was part of a larger reward that also allowed fans to attend a game in the present while also attending a reception with Maddon afterward. Then, as said, there were the Mets last summer. The Marlins are once again selling tickets at face value for the game that's already occurred, but at least this time around, they're attempting to make a profit off of their own player. It's progress!