Import the priv. key to blockchain.info and sign a message from there.
OP sent me his private key but based on seclog,
November 20, 2015, 06:39:07 AM - equator - password changed
. There was a a huge gap between post and the gap got smaller after 20 November, the same day the password was changed. I'm waiting for an email address from OP.
I dont think any other info. is required if OP has given you the priv. keys ?
unless the new owner has a buying "signed message".