I agree that small businesses would be the right starting point. I think you need to identify types of business where a CRM + inventory mangement is all that is required. Then what sets your product aside from the other CRMs with inventory management (which often have many other features) out there?
There is bound to be a gap in the market as most of the popular software is a "one size fits all" so will not be a best fit for some businesses. If you know what businesses you are targetting it makes it easier to decide where to advertise. The collaborative list of products would make it easy to get up an running and is a great USP although would businesses want to share their product information?
I also feel like business owners and sales reps are not going to be on/expect an ad for a CRM on Facebook.
As for ideas for advertising: Cold emailing/calling, LinkedIn ads, Monday.com did well with their Youtube ads, exibiting/visiting trade shows for target businesses, networking events. These could all be options to get some initial customers.
As with lots of internet advice, I am not an expert but do have some experience in the area. Good luck with your app.