I wouldn’t suggest buying lists, firstly it’s illegal in a lot of countries to mass spam people that haven’t opted in.
But also, it won’t have as good of an effect as building up a quality list that have found your site and signed up themselves.
You’re probably better off having a newsletter or sports tips or some other useful section of your website that offers a service that people would genuinely sign up to read.
Double opt-in doesn’t mean that a person has agreed to receive emails from everyone that is gambling related, or from everyone that has purchased their details from a list.
Double opt-in means that someone has entered in their details on a sign up form (the 1st opt in), they then receive an email confirmation that they have to click a link on to confirm that they want to receive the emails to prove that the owner of the address really wishes to be on the mailing list.
Without the double opt-in, anyone could just enter other people’s addresses in to a mailing list claiming to be that person. But if they don’t actually have access to that email address they can’t click the confirmation link on the email.