While it's true that anyone could make up a serial number, if on request, they can not produce any documentation that an aircraft is being built, they lose their reservation and any money paid in. Another possible add-on is to limit the number of reservations that a non-manufacture can reserve to a small number. Maybe 5. Not sure why person or business would need to reserve more than 5 N-numbers.
I also think that the cost of reserving the number should be higher. I can't remember what I paid to reserve my N-number when I purchased my kit, but I do remember thinking it was ridiculously low ($5.00? $20.00?). And here we are.
It sucks that we even have to be discussing this, but this is why we can't have nice things.