Well, I tried it with my iPad 1.
iPad -> Settings -> Store -> signed out of Apple ID
Deleted everything in /Library/Keychains/
Deleted /private/var/mobile/Library/Preferences/com.apple.imservice.FaceTime.plist
-- contained personal iMessage account info
Deleted /private/var/mobile/Library/Preferences/com.apple.imservice.Madrid.plist
-- contained personal iMessage account info
Deleted Diagnostics logs in /private/var/mobile/Library/Logs/CrashReporter
Rebooted
A new keychain-2.db and TrustStore.sqlite3 file were created in /Library/Keychains/ and none of my Apple ID info could be seen in iPad -> Settings -> Store or iPad -> Settings -> Messages or iPad -> Settings -> iCloud
I was no longer able to sign into Messages with my Apple ID, though. I thought it had to do with these deleted files:
/private/var/mobile/Library/Preferences/com.apple.imservice.FaceTime.plist
/private/var/mobile/Library/Preferences/com.apple.imservice.Madrid.plist
so I put back the original files (I didn't really delete them, just moved them to a safe place) and rebooted again. I still couldn't sign into Messages. Kept getting an activation error. I am not really sure what is going on there. Is it because of the files I deleted or something else? I was able to sign into the store with my Apple ID, though.
Now I have a device that I can't use for iMessage and it wouldn't be very ethical of me to sell it as jailbroken without disclosing that iMessage may not work until the buyer does a complete restore, losing the jailbreak.