Is it a good idea to sell your first small batch of functional prototypes as-is while still in the market research stage to get user feedback?
Depends on what you are trying to prove. If there are questions as to whether people will pay for your product, then yes, proving that that is the case with a functional prototype will go a long ways. If, however, you are trying to get feedback, build up brand evangelists and learn, then selling your prototype is not hugely important.
On the other hand we also got interesting support headaches, as the final production version includes all the feedback and thus is completely different in overall architecture, hardware and software and thus probably only thing that is compatible are pinouts of I/O connectors (and voltage levels on them are only mostly compatible).