I would argue that validating your idea should be the absolute first thing to do. I'm not sure how you would fit marketing into the validation process, but bottomline is that you should understand customer needs very well and validate that a good deal of them would love your product enough to buy in before you commit even a single line of code.
I got by the idea that validation is the first step, basic marketing the second and building an MVP is third. The second part is what surprises most people as they would start building at this point.
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