An existing domain name is one that has already been registered by somebody else. However, if a website hasn't been built on it, then in some cases the owner may be willing to sell it. Oftentimes purchasing a domain name is simply a matter of finding the owner's contact details and sending him an email.
Go to Whois.com and enter the domain name that you are interested in purchasing. The site will tell you if the domain is taken. Click on "Who is" and enter the security code and click "Search."
Wait for it to return the contact details of the owner. This will include a phone number, email and physical address.
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Send the owner an email and ask her if she is interested in selling the domain name. If the domain goes to a parked page, or to a registrar, the chances are good that she will be.
Use an escrow service like Moniker.com. If you successfully negotiate a deal, then you should use a service like this, as it will protect both you and the seller from fraud. The fee is 3 to 6 percent of the transaction cost.