Not necessarily. If your solution is solving a real pain point or is creating something really valuable for your potential customers, you can ask them to pay upfront even before the MVP is ready. Treat them as your extended team taking their inputs while you build the product. Easier said than done but its the best validation you can get. You may have to grandfather them on pricing though.

Have a look at this mixergy interview for some ideas/inspiration - http://mixergy.com/sam-ovens-snapinspect-interview/

No. You can presell at a lower price though. The objective is to take cash even if smaller than what you would charge if the product was already built. If people can pay and wait for the product, it means it really solves a problem for them and your chances of succeeding are much higher.

I did this for my product as well. It's paying the bills for me despite not even being done yet.

Think of it as a form of freelancing where the customer just wants some end result but doesn't care about the code itself.