Here is some tips from digitalpoint for buying a website:
Do: Check other websites such as Sitepoint or WebMasterWorld or some of the other trading forums. Chances are you'll find the exact same listing. Read it, if it didn't sell on another forum, there's more than likely a reason why.
Do: Check this person's history. How much reputation do they have? What are their last 20 posts like?
Do: Check the site statistics. Go on to Alexa, what's the ranking? Is the website indexed in Google? Does it have cached pages? Is it indexed in MSN? Are there backlinks? What's the PR?
Do: Check if any of the content is copyrighted, or stolen. Choose some random phrases from the articles or content, then Google it with "quotations". Does it show up on other websites?
Do: 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?
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?
The buyer should be able to ask as many questions, stupid or otherwise. If the seller can't answer these honestly, or without hesitation. Take note.