Contact the sender if you can but don't be fobbed off to send it to the correct buyer the seller must send you the money to post it back them and they post it on.
Or repack it write UNKNOWN RETURN TO SENDER on the front and the return address on the back and hand back to the postie or PO.
There is a scam it's called a Brushing Scam, Google it and see if it applies here.
@jmfw127