The solution I originally came up with would be to run a POS system which is free for shops to use, but to take a SaaS approach to the software tools shops are given, and give them an opt-out feature which is to make their stock room publicly available via an API.

For the SaaSy-ness, you'd want to offer different things that the POS system can enable with the data it has - insights into sales, and as a CRM tool, essentially. All-in-one - give them the hardware for free (but on a lease) and charge for the software, make the API publicly available and charge companies to use the API. Maps-type-applications, SatNav companies, and so on, could pay for this.