How to auction off a domain?

You could go to Sedo and paw through similar domains to get a feel for asking prices (just type your domain or keyword into the search box and it will spit up a list). Some will collect a lot of bids; others will languish for years. Most on the list are abandoned domains that are now being squatted and hope to get a good price, or were squats to begin with.

[I have a Sedo account so I can root around the whole site, even tho I am not real likely to buy or sell more than the odd domain or two.]

As a general rule, .com have value, other suffixes not so much.

GoDaddy has a domain auction thing, but you must pay to join.

Some general info:
http://www.domaintools.com/learn/how-do-i-sell-a-domain-name-423/
and
https://en.wikipedia.org/wiki/Domain_name_auction

Age doesn't mean much. There are domains languishing on the auction lists that go back 15 years or more. Value is mostly in either existing high traffic, or fairly generic (easy for an average person to find via search engine). Frex, hiking.com would bring a whole lot more than johnnyshikingtours.com

Web Hosting Talk is a great forum for such stuff. Frex, on domain value calculators:

http://www.webhostingtalk.com/showthread.php?t=1060547

A few that probably put an optimistic spin on value, but anyway:

http://www.alexa.com/
http://www.trafficestimate.com/
http://www.worthofweb.com/

Just be sure if you give it up that you will never want it again, cuz the moment it lapses, it WILL be squatted, probably forever. Bulk domain squatters pay almost nothing and many own thousands of unused domains -- and for those that actually attract accidental hits, make a killing on advertising on the landing page. Those domains then accumulate value... mostly to other squatters. (There's a huge forum site dedicated to this.)

A good use for an otherwise-unused domain is for a permanent email address for yourself, so you need never again rely on Yahoo (with its spasms of eating mail without a trace) or GMail's questionable privacy. Find an inexpensive host with good mail service, park it there, and use it. I have one that mostly gets used that way, rather than for a website.