Congratulations on the sale and it sounds like it was a great learning experience.
edit: If you don't mind sharing, how did you end up finding the people who were developing their own sites? The post mentions customer engagement but not how you found them in the first place.
I basically ended up doing no marketing for DomainPolish whatsoever besides a few posts on my blog which were popular on HN. A few other people tried out the service and wrote popular blog posts as well. I think doing the thank you videos in the beginning was key because it gave people a huge incentive to tell their friends about the site.
I wish I had more to say on marketing but that's something that I'm working on a lot now for my new startup. Hopefully in 6 months I'll have some more lessons to share :)