I believe what you say is directly contradictory. I'm suggesting you definitely not get it working before you try to sell it.

For most product ideas, the key question isn't, "Is this possible to build?" It's, "Will enough people buy it to make a sustainable business?"

That is to say, the greater risk of failure comes from lack of demand, not lack of technical competence. The greater risk should generally be addressed first.

This is compounded by the fact that it's usually much easier to test the "people will buy X" hypothesis than the "I can build X" hypothesis. Or, put differently, the cost of reducing market risk is usually lower than reducing technical risk.

Furthermore, even if we're in the rare case of working on something where technical risk is significant, we generally don't need to get a real, working product to reduce that risk sufficiently. Some technical experiments are generally sufficient to put us back in the realm of, "Yes, we can make that product."

Slow down. We are both right and both wrong. Figuring out how the reasons apply to you is half the battle.

As a technical person you are better at getting it working so spending a couple weeks getting a working prototype is worth the effort, it might be ugly but it builds trust that it can work. If the idea is radical your potential investors/customers won't believe it is possible.

The real problem is until you have enough paying customers to pay everybody you are living on borrowed money. How you best borrow the money is the only question, and the answer if different for every situation. Some ideas are so obvious when seen that you can get millions from investors. Some ideas will get the "that ain't possible" reaction until you show them the finished product when they will beg you to take their money. Some ideas look cool but nobody is willing to actually pay for them. Some ideas are cool and will generate sales - but never enough to pay for all the people required to make the product. You have to figure out where you are.