I was running it with European city and town geos. We just had simple landing pages offering the service via the index page, didn't even advertise for the batch of contracts we received. Some were word of mouth I think. Schools, shops, a zoo, etc...
Never tried it with non geos.
If you have the right domains, like geos for example, you could really setup a large scale operation I think. Obviously you would need some solid infrastructure in place before hand, and enough staff if needed. I haven't tried it with non geos, I think that would be way harder to do.
I did that too for a half dozen domains. Paid way more than parking. But am clearling those up for some PPC / direct ad driven sites. It was all part of a test phase I tried out for a little while. In the end I learned I didn't want to deal with it.
I had all the domains anchored to a central hub website, where all the domains available for lease were listed, plus contact info, etc. The visitors hit the landing page, saw the available service ad, then clicked the link to the main site, and from there the order was processed.