I mean, iterate on the product until it sells like hotcakes that's your validation. Then you'll spend the rest of your days trying to STOP the torrent of complaints, feature requests, etc. Better yet have actual experience in the industry that the product is targeting rather then expecting that industry to go out of its way to tell you what to build.
Ideally you have orders for the product before you start building it, don't put all your eggs in if I build it they will come.
Not really that unambiguous depending on the market and/or product. You could make the argument that these HIV drugs are miraculously helpful! As you jack up the price. People keep buying it at any price, must means its fantastically effective, right?
Even in less extreme cases, it may be just that you have extremely good salespeople / marketing team. With good enough sales, you can sell any kind of garbage, and in many markets it might be unlikely that complaints of existing users will reach or discourage new ones.
Ideally you have orders for the product before you start building it, don't put all your eggs in if I build it they will come.