You can turn then into content sites, set up an actual website, or try to sell that site elsewhere. You may even luck out by selling a site that's up and running and full of great content already. In this case, you can ask a higher price. Or, you can set up a shared hosting or direct hosting website. Shared hosting and direct hosting websites can be profitable too.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.