Check list :

Check this person's history and how much reputation do they have.

Check the site statistics, Alexa ranking, indexed pages  in Google, cached pages, indexed pages in MSN and Yahoo, backlinks, PR.

Check if thecontent is copyrighted or stolen. Choose phrases from the articles or content, then Google it with "quotations".

Ask questions :

How old is the site?
Was it originally yours?
Did you buy this site?
Why are you selling it if so?
How much traffic do you get?
How much traffic do you get from the US?
What scripts is it running?
Do I need a license for these scripts?
Do I need to install GD Library, MySQL, Image Magick, etc?
Is it your script?
Will I have full copyright to the scripts and content if I buy it?
Is this content unique?
How much does your hosting cost?
How much resources does it use?
What's the income for the past 4 months?
What ad networks do you use?
Do you have a Google Adsene account? Where you banned?
Will you help me set it up?