The best way is to look at the keywords in use for your classified search niche. You can then build domain names by using these keywords indirectly.

You just put your question in foroum sites.

Example of 1 classified site- biggestclassifieddotcom

Ads4free

youandme

alliswell

may be allin1classified.com

http://www.thelearndoshare.com/

Hello,

Please suggest me some new classified domain names. I want to registered new domain. I m creating classified directory for posting free ads. Please suggest .com or .in extension domain names.

Thanks.