Broad topic onboarding "I will not promote"

Anyone got any tips when you're start-up is in a broad category? Say as an example I wanted to create a "amazon" esc thing (not what I'm building just for example purposes) But where it's a generic marketplace: The advice for start-up outreach is "Go to where your ideal customer is.." okay my ideal customer is a generic consumer.. they're bloody everywhere. Do you; Purposefully try and push the business into a niche so you can then more specifically target potential users (or) do you just try cast a wide net doing generic ads, generic traditional marketing and just attacking general buying and selling groups that allow promotion and hope you get people onboard?