Is selling on a marketplace better than your own store?
Better for finding demand, worse for building a business. The marketplace hands you traffic and keeps the customer; the relationship is the asset, and on a marketplace it is not yours.
The reasoning
The workable answer is both, deliberately: the marketplace as a discovery channel and the store as the place a returning customer is taken. What must not happen is a decade on a platform that can change its terms and take the business with it.
What most people get wrong
The number that decides whether a store works is not revenue, it is what is left after the cost of the goods and the cost of getting the customer. Plenty of stores with impressive revenue make nothing.
Repeat customers are the whole game. Winning a customer costs money; selling to them again costs almost nothing.
Most of the money in ecommerce is made in unglamorous categories bought regularly, not in the ones that photograph well.
How an app changes it
This is where an app earns its place — it exists to make the second and third purchase easier, which is where the profit is.