Steam

I think the issue is less the existence of gambling sites, and more the fact that underage gamers are often the target of the sites. An age verification for trading would be the easiest, but Valve has taken a hard stance against collecting identification for any reason. The age verification could come from the websites but that seems very unlikely since the websites are often illegal. If enough countries (especially America) legalized online gambling but required ID verification, the sites may be more likely to implement it, but that is so far of a scenario there really is no prediction.