Perhaps if you are a buyer of this data you have similar, if smaller, databases. You could then say, here's ten thousand hashed credit card numbers from my coll

Perhaps if you are a buyer of this data you have similar, if smaller, databases. You could then say, here's ten thousand hashed credit card numbers from my collection. Give me the full data for ~1,000 of them. If their data lines up with yours that's a good sign. This wouldn't be a perfect method, but if the seller could do it then it would increase my confidence a lot.