Can you believe it’s been 12 years since J.K. Rowling’s first Harry Potter book was published? And nearly a decade since the release of the first movie adaptation? Enough jokes have been made about the maturation of now-grown stars Daniel Radcliffe and Emma Watson, and with two films still left  in the pipes, they’re sure to keep coming for at least the next couple years.

But what of this trend that compares eBay sales between July’s most recent film, Half-Blood Prince, and 2007’s Order of the Phoenix? Below is a two-year Terapeak trend for total sales in the Harry Potter Collectibles category:

Even if the text is a bit hard to read (we’re working on fixing this), you can see pretty clearly the two major sales points for the series: release date and the holidays. And while the disparity between the holidays (the two middle peaks) can be attributed to Order of the Phoenix’s DVD release, the difference between the two films’ theater releases is more than 2:1!

It should be noted that July 2007 also saw the release of  Rowling’s final Potter book… but the decline in sales of related collectibles is still quite shocking. Is it a case of fans simply losing interest in the series? Is it possible that the series lost its luster when the books stopped coming out? Heck, we’re even wondering if it’s some kind of Twilight effect!

The next Potter film is slated for release in November 2010, with the final installment coming July 2011. Before then, however, comes Half Blood Prince’s DVD release… which will no doubt make for another interesting comparison!

Summer blockbuster season is well under way, and so far we’ve seen the release of such box office giants as Star Trek, Transformers 2, GI Joe, and Up, all of which seemed near-perfect from a merchandising standpoint, and all of which we’ll be re-tracking once DVD-release-time rolls around (holidays, anyone?).

But later this week comes the granddaddy of them all: the sixth film installment of the adventures of Harry Potter, the half-muggle half-wizard who’s an industry all his own.

It should go without saying that now’s a good time to gather anything Potter-related and see what it’s going for. Just take a look at the following 30-day total sales trend for “Harry Potter” (and keep in mind that this data extends only to July 11th — nearly a week before the movie’s release):

Now you probably can’t see clearly (unless you’re really good at squinting), but that graph shows Potter selling tens of thousands of dollars’ worth of goods every day… and it’s not such a stretch to imagine the release of Half-Blood Prince shooting that number up significantly higher.

And to show that it’s not just sellers affecting Potter’s eBay ascension, here’s a similar 30-day All-Sites trend for “Harry Potter” bids per listing:

We’ve got a few Harry fanatics in the office here ourselves, but those of us who aren’t waiting in ticket lines all weekend will surely be tracking this up-tick going forward…