Most startups don't have product market fit. Further, most startups make things no one wants. The product might sound good to you and you will think "oh I can sell this", but you will work your ass off for no results.
They you may go back to the client and say "Hey the market doesn't want your product, they want this instead". Now you are doing free market research for them, and most likely, they won't listen to you anyways.
You may think "okay I need to be good at identifying startups struggling to sell who have products people want" and then you may realize there's a whole industry of VCs trying to do this, that mostly suck at it.
I think you'll find that young companies with products people want who are struggling to sell is extremely rare.
There's a reason the business model your suggesting isn't more prevalent with startups, and it's because sales skills matter far less than market fit.