I'm pretty sure it was only sold to ticket holders and there wasn't anywhere near 510 seats.
But yea, it will sell out instantly.
Alamo/Mondo usually does it one of two ways: You pay for the poster with your ticket (so maybe like $60 for instance) or they just sell them in the lobby. They also usually sell extras in the lobby if they're included in the price. They maybe did not here (can't say as I didn't go), but if they were selling them as well and this was a halfway decent Stout then they'd have all gone.
Could be that he has an agreement though as Rinn mentioned.