You already have valuable curriculum.
Most of it isn't packaged yet.
The gap between ‘I teach this well’ and ‘I have a sellable curriculum product’ is smaller than most teachers think — and almost entirely about packaging rather than creation. You've already done the hardest part: you've worked out how to explain something complex, which examples unlock understanding, which misconceptions need addressing early, and which activities produce the outcomes you're after. The curriculum exists. It just isn't formatted as a sellable product.
What do colleagues
already ask you for?
The fastest signal of market value is what your colleagues already ask you for. If other teachers in your department regularly borrow your Year 11 organic chemistry revision materials or your A-Level essay scaffold — these are candidates for your first listing. Existing demand from colleagues is the closest proxy for marketplace demand.
Don't compete with
established resources.
Before investing time in packaging a topic, search the marketplace for existing listings. If there are 20 well-reviewed resources for GCSE trigonometry, your resource will need a distinctive angle to convert. If there are 2 resources for A-Level Further Mathematics complex numbers and both have minimal reviews, you have an underserved niche with clear demand.
Not a full course.
A complete, publishable unit.
Your first listing does not need to be a full course. The minimum viable curriculum product is a complete unit plan (4–6 lessons) with: a clear scope statement, learning objectives written as student abilities, teacher notes that a teacher who has never met you can follow, student-facing materials, and a formative assessment. This is publishable, adoptable, and demonstrates your quality to future buyers.