Betcha he stashed 'em in the back of his Woody while hanging ten with the Wilson Boys and Jan and Dean.
Don't know about this guy, but I talked with a different seller of these posters... he was a kid handing out fliers for this film back in the day. He figured he must of gotten lazy one day and stopped early -- because late last year he found a stack of like 52 (or something close) in box in his parents garage. They were all printed identically (Santa Monica Civic Auditorium). He even had a few handbills with press quotes and such. He was asking $100 a pop, but apparently he got cleaned out of his stash before I could get 1... or 10

Something similar probably happened here as well. I sure the stash I mentioned above will turn up soon too. If they keep selling them at this rate... they'll be back down to $100 pop in no time. And we should probably all buy one once they do...
They will eventually run out and someday on APF-brain-implant-chat we'll all be telling newbies about "that sweet stack of Endless Summer posters that turned up back in 'aught 11. Man you should have been there!"