Thats a question every dn owner asks themselves daily...lol!
Its like whats the answer to the universe.....43(from memory)
Firstly, you should determine what you think you will actually want sell for, just like selling anything in business.
The market may see value or it may not.This may change at any time, so be prepared to wait.
-You can get an appraisal on dnf which may be helpful.
-Look for multiple places to offer it(find where buyers are located)
-Look for trends that point to possible value
-Actively search for market $value indicators that may relate to your dn(s)
-Know what realistically the range may be for your sale potential before offering it for sale in a "reseller/trader" market like this one.
-Different type of dn's have different appeal/presumed market value, ask questions, review many other sale sites and gather your own ideas about potential value to "end users/business types for a better understanding about where and when to offer it for sale.
-search for ovt/keyword results across search engines to see if it adds value to your dn(s) by what consumers/visitors are looking for.
-There are many factors that may or may not influence the potential market percieved value, so spend as much time as possible watching what others are doing/selling.
Ask lots of questions and listen to the market overall, not just a few peoples interpretation of value. The market can change at any time, an "end user" might want exactly what you have andmay also be the only person interested.
Test the waters & ask lots of questions.Be realistic with your expectations based on the knowledge you acquire relevant to the market and your domains.
Sell for what makes you happy, if that doesn't work join the rest of us in Dnfantasyland.com and keep buying more until you have so many to manage that you sell some to buy more!
Also domaintools.com is a good resource to use.