I hope I've understood you correctly - you have email lists of people that have reached out to you and offered guestpost services.
I see several options, depends how much time you want to invest:
1. Probably find a couple of SEO tools that have an affiliate program (register there ofc), write a couple of review blog posts for them, and then market the posts to those emails. I guess they're a relevant audience, since they do onpage/offpage SEO. Only thing that concerns me here is their quality, since they seem pretty low budget to me...
2. Maybe just start hand picking some of them and working on scaling your sites, and eventually when you get enough of traffic earning opportunities will probably start coming in (sponsored posts, paid banners, promotion blogs etc.); or, again, register to a bunch of relevant affiliate programs -> start scaling traffic -> eventually you'll have passive income;
3. Create your own Offpage network and map out a monetizable service in which you are a middle man and skim the % of total earnings, while connecting both parties (link builders and other website owners)
Hope this somehow helps, good luck!