I fully agree.
The seller knows nothing more than the size and the color.
The Joule thief is a demo, so no critical parameters.
But still, how can I order the same ferrite cores from some other seller or from the same seller later on, if they cant tell me more than the above?
It would be wonderful to differentiate by color, at a minimum. But would that suffice?
+++
An alternative might be to use transformers in place of the torroids or to find somewhere a circuit using linear inductance. I would avoid that since it will certainly lead to more complex circuits.