When you have secured a deal to purchase a domain name, especially one that has significant value, you need to do your due diligence. The most important research you need to do is to ensure the domain name is owned by the party that you think owns it, and the second is to make sure traffic/PR/directory listings and all other promises can be met.
In my opinion, ensuring a domain name is owned by the person/company you believe owns it is the most important thing. Having a Domaintools account allows me to search the Whois history for a domain name. If you are ever unsure of whether a domain name is rightfully owned, you should contact the previous owner to confirm that the name was sold. Phone calls are generally the best way to confirm it, although having an email record is also important.
You can sometimes use the Whois information to make sure a domain name is able to be sold by the contact person. Because some companies use their web developer or tech support company (or person) as the contact, even though you are dealing with the contact email, that person may not have rights to sell the domain name. If you are unsure or have questions, visit the website and contact the company directly (by phone). You can also Google the registered owner and contact the company c-suite offices or the executive listed.
Verifying other things like stated traffic, page rank, inbound links, or other promises is also important. You can ask to test traffic on your servers for a week to try and confirm that it’s “real” traffic and not bot traffic. You can also use free traffic check services like Alexa or Compete. There are a variety of ways to check page rank (like the Google tool bar or many websites that can be found by searching “page rank checker” in Google). Finally, inbound links can be checked on Yahoo’s Site Explorer.
Do your due diligence before buying a domain name. A few extra minutes of research can save you a lot of money.