Selling old domain or using for 301?

I have a couple older domains I want to sell. (between 3-5 years old) How do I know what the real value of them are. Godaddy valued both at $395, an offsite valued one at $960. I think its because its been a blog set up on that one and I get traffic to it and a whole lof of comments to it. Is there auction the best way to unload or posting in sale forums and such? Can you point me in the best direction because I now know that sometimes the older domains are more valuable for the 301 with SEO but not sure how to use it too my advantage..

OR is there a way I can somehow pass the age of them to a new domain I have to help build the age of it.

Don't go by what Godaddy or other sites are valuating the domain names. If you have a small profitable niche in mind, spam the hell out of those domains (profiles, guestbooks, comments with atleast 50% brand anchors and naked URLs) and 301 to a new brandable domain. I hit page #5 doing it for a medium competitive niche. I need to 301 a few more domains to it.

Ok, meaning just do a boatload of backlinks to the older domain name but have it set to 301 to my new site?