I sell through domain marketplaces like sedo/afternic/flippa/dnforum. I also reach out to startups directly that correlate to domains I own, it's a very effective way to sell domains. You can also develop them into SEO/amazon referral sites for product-related domains. I know a lot of people that make thousands per month off of pure referral sites, mostly auto-generated.
It's not my primary income, it's something I do on the side. Once you've bought a domain, you don't need to do much work after the initial parking setup, which takes about 5 minutes.
