Your domain name registrar may provide you with a free one-page Website tool, which you can use to create a “this domain is for sale” landing page. Alternatively, you can create a single page (perhaps a hidden page hanging off one of your existing websites) that indicates that your domain names are for sale. You can then forward all of your domain names that are for sale to that one page.
Hundreds of thousands of domains expire everyday. While most of them are trash, quite a few are aged domains having excellent backlinks. These can be easily filtered using metrics from Majestic (Trust Flow) & Moz (Domain Authority). These are great for your money sites or for building private blog networks. Some of these domains have natural organic traffic, that can be determined by their SEMrush rank or the SimilarWeb data. These are great for domain parking or for promoting affiliate offers.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.