damian,

it would be hard to sell gardening items if you build a money making list yet there is some cross over in the money making field niche, internet marketing has multiple steps - outsourcing, listbuilding, SEO etc.

so you have some leeway within a niche

determining value - would someone want to read, listen to or watch what you put out and would after consuming your free giveaway, would they want to come back to you? that's vital in determining "suitability"

many "gurus" actually use some of the course material in their free stuff.