> This begs the question... what about products not sold?

You're liable for interference, but you don't have to get part 15.
As a company, prototypes can be distributed, but they must be marked:
"PROTOTYPE NOT FOR SALE", I think you have to add some text about not being
in compliance (or at least untested) with part 15

Those prototypes have to be rounded up in the end and either certified, or
retired, or you're responsible for them.