I agree, plural is a must for this type of name, for example namepro.com, would to me anyway, indicate software. You could develop an App that specializes in vintage registrations and develop value for the name. A google search on Mature Domain brings up some interesting and technical content. MatureDomains.com resolves to typical, bland laziness. Offer the owner of MatureDomains.com $500 let him talk you up to $1000 tops and then you will have the both the singular and plural.
To me personally it is worth $2000 undeveloped.
FWIW Nokta bot valuates this one at $3,611.
Pestibot valuates this one at $70
Regards
matt