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Is there any way to solve this transaction problem without a mutually-trusted third party?
It would be necessary to reduce the criteria used by the purchaser into algorithmic form. Write a program which would take the data and produce a yes/no answer. The program has to be such that even if the seller knows the program, he can't produce fake data which would fool the program. Then the seller can commit to his data and produce a zero-knowledge proof that the committed data satisfies the criteria in the program. This can be done reasonably efficiently for moderate sized programs. It will leak no information about the data other than the yes/no answer from the program.