My concern is the accounts are hijacked and paid with a stolen credit card or the like.

Many scams out of China and growing as more n more discover how to get quick cash over the net.

Of course it could just be genuine.

Whats your plan ?

When you receive it back, refund then ask buyer to repay changing address in checkout ?

Personally I would probably have sent back an immediate message saying, sorry but already sent.

I'm sure they could have arranged something from an address they haven't been away from for long.