The annual Penny Arcade Expo in Seattle is a fascinating study in the laws of supply and demand. Passes to the three-day event traditionally sell out quickly, creating a secondary online market of real and counterfeit passes to the show.

During the conference, you can gauge the changes of success for an upcoming game based on the number of hours that people are willing to wait to get some hands-on time or hear from the creators.

But the economics don’t end when the show is over. Some of the items given away by companies at PAX this year are already being offered online at margins that can only be described as healthy.

Exhibit A:  PopCap’s Plants vs. Zombies Wall-Nut hats, which were given away for free at PAX, are now being offered on eBay for anywhere from $10 to $40. The giveaway attracted a long line at the show, and the company ultimately had to turn people away when it ran out.

Jeff Green, PopCap’s director of editorial and social media, wrote on Twitter this morning: “I suppose it’s ‘flattering’ that our free hats are going for $30 on eBay. Still: Kinda LAME to sell what fans waited in line hours for :(”

Chris Kohler of Wired GameLife pointed out in response: “Well, from the seller’s side, they’re mitigating the cost of their PAX trip. And buyers get the hat without going to PAX.”

Responded Green, “Yeah maybe. But the ‘markup’ from free to $30 is a lot. Plus we had to turn away lots of genuine fans who WANTED the hat to keep.”

One hint that there were lots of non-fans in line: the references to wanting a “potato hat.” Green wrote, “It’s a WALL-NUT, people. Know your goddamn PvZ pleez.”

As Green noted in a separate tweet, PopCap recently launched its own PvZ store, so it’s hardly oblivious to the fact that there’s a market for this stuff. But it’s too bad there isn’t some way to ensure that real fans of a game get first dibs at an event like PAX.

Another Twitter user suggested to Green: “Next year’s hats need embedded remote-kill microchips so you can incinerate them from afar if they get auctioned.”

“You’re hired,” Green replied.

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