Automated API Testing

Postman founder here. Postman do automated API integration testing using the newman command line companion [1] for Postman. You can build and run your tests in Postman and then integrate them with your CI system.

1. Newman: https://www.getpostman.com/docs/newman_intro

To me, the best reason for a developer to build this sort of tool is because it solves their own issue [or by proxy that of their customer/user/client]. The reason is that there is a natural pressure to build the least amount of functionality that gets the job done and there is less tendency to speculatively develop features for which there might be little or no value based on imagination about how a not yet existent user might use the tool.

The best place to sell such a tool is to organizations that are unwilling or unable to develop such a tool in house but have a need for it. That's B2B sales and determining the demand in that market is probably best done by talking to people in relevant businesses where there might be a need.

Which is a round about way of saying that that the number of responses in this thread may not be indicative of the size, potential, or enthusiasm of the relevant market.

Good luck.

https://apiblueprint.org/tools.html# lists a couple of testing tools. There are also tools that work with the Swagger/OpenAPI specification https://github.com/jolicode/jane-openapi If I remember correctly Postman can export test cases as well, so building a service that runs those should be possible, similar to saucelabs.com/selenium.