Well, you could always ask him for a bit of time to decide, and say you'll make your mind up on Friday, as it is a large amount of money to pay.
He'll usally agree to this, if he's legit. Anyway, after that, when friday comes (make sure it's past Blizzards Phone Hours) and speak to him, saying you've decided you'll buy it, on ONE Condition : You get to have a 5 minute 'test drive' on the account.
Most of the time they'll agree, and give you the Username + Password (Note : You MUST have a working Battle.net Account)
Once you recive the Information, quickly log-in to your Battle.net account, Click "Merge now" or along those lines, and type in the Info... Now, you'll have control over the account for the whole weekend.
The Username will change to the email for the Battle.net account, same with the password.
hope this helps.