Using a domain name escrow service would be the normal way of purchasing an already registered domain name. But one thing you may want to do first is check on how easy it will be to change the legal registered owner of the domain name. For example, if the domain name ends in .co.uk you will need to pay Nominet a ten pound fee to have the registered owner changed. I would in the domain industry, so if you let me know the end suffix (.com, .biz, .org.uk) I might be able to let you know. I hope you wouldn't, but please don't publish the full name of the domain name you're trying to buy.