How much worse is it to use weapons against your own people as opposed to your neighbours, or people a bit further away?
I actually think that if a country uses arms against the country that supplied the arms in the first place thereis some degree of poetic justice to it, on the contrary.
Do you expect an unwritten code of conduct that weapons will not be used against the supplying country but should only be used against other countries? Isn't that, even if it could ever be implemented and adhered to, extremely cynical? That would make selling arms a way to defend yourself: 'go shoot other people, kill them all if you want, but don't touch me...'
I think that we should first look at weapons that have been outlawed or should be outlawed. Think of Israel dropping cluster bombs in th elast 2 days of their invasion of Lebanon, think of Israel (I'm not picking on them, it's just more recent) allegedly using DIME weapons that destroy animals and people but leave infrastructure intact, think of the damage land mines cause years after the conflicts have ended...
I think that the way to outlaw these would be to hit countries where it matters, in the pocket. If you sell landmines to country X and country X uses them, you will have to pay for the clean up. The directors of the corporations doing the sales and the heads of state and military of the country using the weapons should be personally responsible, declaring the corporation bankrupt would not be an option. If you supply cluster bombs, then you will have to pay for the clean up as well as the loss of income and cost of relocation of the residents of the affected areas.
Basically I am saying that the true costs of conflicts must be borne by the perpetrators and their suppliers.
This could be enforced via trade boycotts, just as those used against Iraq prior to the war, ensuring that they couldn't produce any WMD.
FInally this should be applicable to every country and corporation.