You are right, update is no problem, of course. However, you are buying a machine with outdated firmware and every time you run macOS Recovery over the Internet it is going to install Sierra, even a year from now. Quite a hassle if you do this a lot (which, of course, does not apply to most people). macOS Recovery over the Internet is a great feature and I totally hate getting the old OS and having to go through the update process every time I do this.
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Would you mind entering your serial number at powerbookmedic (
http://www.powerbookmedic.com/identify-mac-serial.php) and checking which OS they say? If they say Sierra, maybe they even get this correct and would be a reliable source of checking the serial before buying. Confirmation with another purchase with High Sierra would of course be needed.