I have been thinking about this issue, and have discussed it endlessly over nearly 30 years.
There are three basic factors here:
Enforceability
Equivalences
Proportionality
Enforceability is a problem for several reasons. The biggest one are that the majority of the sales of gas appliances are not to registered installers, but are still perfectly legitimate. They are either sales down the supply chain (ie, Manufacturer > Distributor > Merchant) or sales to organisations who quite reasonably want to purchase the materials, despite having no intention of fitting them illegally. A large proportion of our sales are to Councils, HAs, Universities, MOD establishments and a whole range of other public and private non-GSR organisations. There is also the issue of whether a particular item is a "gas product" or not. Lots of things are dual purpose.
Equivalence presents another issue. If we seek such protection for the sale of items related to our industry, we must expect other industries to make similar arguments for equally draconian regulation. So sorry, you can't wire your own plug, maintain your own car, mend your own roof, or even use your own sex-toys! Poorly maintained cars kill FAR more people than poorly maintained gas appliances, and more people are hurt doing DIY up ladders on their own house than are affected by CO poisoning. Hospital A&E departments regularly have to remove objects from orifices in which they have no place being, and this would never happen if they were only sold to competent sex-workers.
Finally, we have proportionality. Starting from the point that "its a free country", as a society we accept a range of limits on our freedom for the collective good. We accept quite stringent limits on things like the practice of medicine, ownership of firearms or explosives because of the high likelihood of frequent unpleasant consequences if we don't. Moving down the chain of potential risk, we regulate bus and truck drivers more than car drivers because the consequences of their failures are greater.
At the bottom of the regulatory pyramid are the activities that merely require you to be a certain age, and consenting - like drinking alcohol, smoking, having sex or joining the armed forces. They key to this is proportionality - the degree of regulation is proportionate to the frequency and severity of the harm that would ensue in the absence of regulation. Increasing the level of regulation on the gas appliance market would simply be disproportionate - the benefits do not outweigh the costs.
And before someone comes out with the old argument "surely its worth it, if just a single life is saved...?" Nope. On that basis we would have a speed limit on all roads of 20mph which would pretty much eliminate road deaths. But the cost is unacceptably high.
So, much as I understand the sentiment, I absolutely don't support legal limits on the sale of gas appliances. Or electrical fittings. Or car parts. Or ladders.
Or sex toys.
I should add that we don't sell anything to DIYers. However, assuming a customer is a genuine tradesman, we also don't try to act as the industry busybody or policeman. Having said that, all our branch managers have a standing instruction to refuse to sell ANY product if they are doubtful about safety, and they will always have my backing if they choose to exercise that discretion.
