Not knowing the source is enough to keep me from buying a stone. I don't have anything that I couldn't locate the source of and I'd hesitate to count the number of stones I've bought. It's a matter of courtesy.

Agree on the cost of finishing - the larger and harder the stone and the less reliable the deposit, the more it costs to do that. This appears to be a common stone with a stiction problem from the demonstration. If it finishes a razor for $15, great. If it costs $90 and has a stiction problem, not so great.