If you have not yet startup, I would strongly suggest launching as the AirBnB WordPress theme SpotFinder.
I have used this theme to launch directories on sub-directories of my main domain in order to shoot up the SERP, but I have friends who have monetized this theme as is.
The greatest thing about it is, you can run a web scrape of a larger non-niche directory to grab every listing in your niche, then use the bulk-upload theme here to upload thousands of listings at once. It is a pain to format the .csv before upload, but you can either have your scrape set up to pull data in the same format, or you can have a VA format the giant .csv files to work with the bulk upload.
In either case, you can have an enormous niche directory in a matter of days.
I hope this helps.
