How I run a bootstrapped marketplace with 11 sites and 5,000 vendors – by myself
limedaring.com
limedaring.com
I honestly don't remember my success rate as it's been a long while, but I want to say it was at least 25%. Keep in mind I was pitching a free product that vendors are already used to (there are TONS of wedding vendor directories.)
As for scaling, no idea. Some CRMs allow you to mass-send email but I worry about those - feels scummy, though maybe I should just get over that feeling and try it out.
Good luck!