It used to be called "vapor-ware" and had a negative connotation. Now apparently it is a popular thing to do. I think it is deceitful and annoys me as a consumer. I will stay away from any company who does this; I will not purchase if they do create the product/service
I know Rob Walling promotes this idea but I would try a different way rather than make people think they are going to a site that has a real product.
Perhaps the question is slightly different.
Figuring out what to build “What is the smallest or least complicated problem that the customer will pay us to solve? ” vs "Offering something behind a buy button that doesn't exist " are two different approaches.
I agree with others that the latter seems a bit negative / underhanded, but the former requires face time and research to hone your product offering. The former is called "customer development", and is a critical part of the "minimum feature set" product definition in many lean startup methodologies. A good article to learn about it can be found here.
I have done such "testing" with my friends. Explaining what the product will do and asking them what they expect about different parts and functions. Asking questions about different variants of design (A/B testing) etc.
Prototyping for proof of concept.
You build a prototype of your product or service and test it to prove your concept.
You can build as many as you wish to see which works better.