Content setup: If your domain has potential to attract good search engine traffic, you might want to set up some content to help facilitate that. If this helps your domain do better in the search engine rankings then it’s a big win regardless of whether you choose to flip the domain only or flip the domain with the content as a “website”. The SEO value will be helpful either way.
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.