But if it required user activity to register for those lists AND you explicitly identified them as for marketing purposes, that seems like you already HAVE consent? I mean, what do you imagine consent to mean other than "an active affirmation from the user that they're ok with this". If it's indeed double opt-in AND clearly communicated, it seems you clear that bar by a mile?
I just found another article however with another solution which may be better. The article suggests instead of saying "Get this free ebook! And p.s. we will send you information and marketing emails about our product." you should say "Sign up for our newsletter to receive information and marketing emails about our product. Also we will send you a free e-book as a gift." It's not as good of a call-to-action, but changing the order does turn into explicit consent for marketing. Source: https://blog.mailrelay.com/en/2017/12/28/new-gdpr#_What_abou...