I had a directory of local farm shares/CSAs for about a decade. That one wasn't super easy to monetize, so I'd pick your niche carefully. It also was a ton of work right around the holidays. It helped a lot of people though.

I'd avoid building software--you can find Wordpress plugins that give you everything you need. That will get you up and running quickly from the tech side.

If you follow my advice you'll spend most of your time doing marketing and sales to try to get both free and paid listings.

If that excites you and you love the space that you're cataloging, go for it! Otherwise I'd avoid.