Clever e-mail scam: Someone bought this in your name

The e-mail appears to come from PayPal, a popular institution often imitated by scammers. What's unusual is that it seems to be a confirmation of a purchase, for more than $400, paid from my PayPal account. There's even a realistic-looking transaction, including the name and address of the person whom you're supposed to assume made the purchase.
Clever e-mail scam: Someone bought this in your name