It seems like many on NP are using Efty to publicize their domain portfolio. An advantage of this is the ability to distinguish your brand as a standalone shop. It makes you look more authoritative imo.
Undeveloped also has its benefits for those who don't want to invest time/money/energy in setting up a site. I like the simplicity of their back end.
If you want to invest some time/money/energy in learning how to setup your own standalone shop, built and operated by you, there are options. I have found Joomla to be a fun CMS to learn (still learning). You would likely need to become acquainted with various technical aspects of web design and hosting, but if you're the type that likes to explore and get their hands a little dirty, it might be a fun option.