Ask HN: Advice on Building and Monetizing Dataset

I'm pulling together a fairly large dataset that I am using for some side projects and I believe it could be valuable to other businesses. Any advice out there on the following?

1. best way to test whether a market for the data exists,

2. best way to sell the data to customers (are there good, established marketplaces for datasets?)

3. tips or lessons learned on designing APIs and running a business focused on selling data.