Yeah, I'm really surprised. I have no explanation for that but my account is NOT for sale! And I changed the password a few minutes ago.
How can I find out wheter I got hacked? I don't have these messages in the outbox, I haven't even used this account for weeks...
You can't know if you were hacked. There is no email notification if someone else logs into your account so you can't just say "I wasn't hacked"
If you weren't hacked that means It was you who sent those PM's which make things even worse.
Now you need to verify if you really are the original owner of the account. Signing a message from an old address would work.