I could be hard to do without access to data and costly integration. And like shorting, the difficulty is how to monetize. It could also be easy to game. Still...
The nice thing about the business is that market (publishing) is flourishing. Not sure about state of the art or availability of such services.
For sales: run it on recent publications, and quietly ping the editors with findings and a reasonable price.
Unclear though whether to brand in a user-visible way (i.e., where the journal would report to readers that you validate their stuff). It could drive uptake, but a glaring false negative would be a risk.
Structurally, perhaps should be a non-profit (which of course can accumulate profits at will). Does YC do deals without ownership, e.g., with profit-sharing agreements?