Make sure he is the original owner of the license, that he didn't buy it used from someone else. If he's the original owner and you trust him you'll probably be OK... but vBulletin.com has a sticky thread about things you should be aware of when buying a used license- wouldn't hurt to read that just so you know the dangers.
Also is it a vBulletin 4 license or a vBulletin 3 license.. if it's VB3 you'll still need to purchase 4 (for close to full price) if you intend to upgrade.
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