I'm also assuming the price does "not" include WIN 7, and you will be supplying your own copy? If it does include WIN 7, then I would tread carefully. Ask for the COA sticker with your product key. If he doesn't have one, then at minimum it could be a Windows volume license. Worse, if its pirated, as MS could invalidate the license while doing a future update, and essentially lock you out until an authentic product key is obtained.
I would also validate Windows in person. If he says its already done, or not necessary, then go into "Start>Control Panel>System and Security" and click "System". Scroll to the bottom to Windows Activation, and make sure there is a Product ID: number listed. If there is none, then its likely a pirated copy of Windows.