Every domain investor should do some due diligence when purchasing a domain name. Generally, this entails a Whois lookup at a site like DomainTools, and it should also involve a Whois History search. If something looks funky in the Whois records, there may be reason to investigate further.
One of the best ways to do due diligence on important domain names is to contact the previous owner. You can easily find out his or her information in the Whois History too. You’ll want to let the person know that you’re calling to do due diligence and you just want to confirm that he or she did in fact sell the domain name to the current owner.
If you really want to pry, you can even ask the price, but most people will find this question inappropriate. You also may not want to know how much more you are spending than when the domain name was previously sold.
Like purchasing art, a domain name’s provenance is important to know. Generally, a domain name will not become more valuable because a particular person or company owned it, but you should know that all previous transfers were legitimate.