I was wondering that too.
Maybe Indy lost it on his way to collect Marion Ravenwood... Or since it was also in India, maybe on the Temple of Doom adventure.
Although come to think of it, I remember buying stuff years ago for some of my little ‘nik-nac’ shops from a company called “Tibetan Traders”... Including crystal and stone skulls.
Maybe this thing is no Mayan antiquity at all, but a modern Tibetan handicraft, and the reason that the owner was so lackadaisical about the damage is that he paid ten - twenty bucks for it.