I agree that your interpretation is right, but "start selling" is an extremely ambiguous phrase here. After all, "a sale" is most often understood to be a finalized sale; "to sell" is most often understood to be completed transaction.

Yes, the idea here is, "understand who you're marketing to and what they're looking for before you start building," and the phrasing used is what it is because that's catchier, but it's still not particularly concrete phrasing.