If you create content, it's always better to sell your own product than someone else's.
But at the start, selling other people's products is a great way to understand what your customer actually needs. You trade margin for information to empower your investment.
The progression — from someone else's shelf to your own
Once you know what they need, build it. Start with courses and ebooks about your own knowledge — they're the cheapest way to package what you've learned. Then move into products.