Imitation alternative-health merchandise, pseudo-spiritual chochkas, and cheap novelty accessories offered at most open-air fairs and by two-bit street vendors in college towns.
It's a take-off on the classic Birkenstock shoes. The original Birkenstocks are high-quality foot-wear worth anyone's money. The fakes for $20 fall to bits in two weeks' time. By extension, any street fair has stalls of well-made goods created chiefly by local craftsmen and craftswomen, and then also those stalls offering vastly inferior import trinkets and poser products fronting to be the real deal.
"This fair used to be good, but now there are mostly fly-by-nighters trying to palm off greasy pizza and birkenshlock."