Ask HN: Advice on creating a POS system?

I've created an awesome database application that is great for small businesses and one of the niches I'm considering selling it to are retail stores that want to put their inventory online.

This market is dominated by POS software systems. Every retail org has one, just about. I was wondering if any of you have created a POS system and whether or not it is difficult to create this type of functionality for an inventory management system.

Anything I need to think about or resources you can recommend?