How do I use iMessages without a sim card?
This is the same set of steps you would use if you had a sim card, it's more about clearing the account that device has registered with the iMessage service.

Try to turn off iMessage completely, totally log out of the system in Settings > Messages.  When you re-enable it, ensure you log in with your own Apple ID.  At this point, you should be able to message to your registered email address.

If that doesn't work, you may need to back everything up and restore the device. This is somewhat problematic without a SIM card, since you may get stuck at activation if you don't at least have an inactive SIM from the proper carrier.