Amateur alert: i am NOT a professional paleontologist; I am a semi retired therapist who happens to be a paleontology hobbyist.
I've seen several things advertised as gastroliths, and the prices seem easily doable.
But HOW in the world is a rock determined to be a gastrolith, and not just a smoothish-rock that's a bit worn?
A rock that has been inside a prehistoric animal (not just dinosaurs, I'm guessing?) is WILDLY cool, but a smoothed and tumbled rock is not.