There is a difference between Product Opportunity Validation (POV)and Product Solution Validation (PSV). POV can be done through customer interviews (without the need of a prototype), while PSV should be accomplished with a working prototype/demo/mockup as you mention.

However, when you mention “sell it”, it implies that the customer actually pays you money for your product. I’m sure there are outlier-stories where clients pay for a “mock-up”, but do you believe this is the norm: that startups can collect payments from their clients with a simple mockup?