You would be best getting some legal advice on this one, your going to be manufacturing or refurbishing a product to sell on to the public which could leave you liable for anything that might go wrong.
If your just selling on a heater then the manufacturer of the heater will be liable for this but again it wouldn't do any harm to
cover your back with a bit of legal advice.
As for what you could get money wise, well for all the benefits hot users like to brag about they don't seem to be willing to spend a lot of cash for the privilege (going by this forum) so I honestly don't know if its a winner money wise, maybe the fact its not a gas system might get some to part with a few quid more.