Export Keychain Items is not enabled from File menu of Keychain Access As can be seen in the screenshot I have selected a bunch of keychain items. So why is Export items disabled?
You can only export one key at a time, or one key and a matching certificate.
It will be encrypted upon export and can only contain one key.

If you need to move an entire keychain to a different computer, look at ~/Library/Keychains - example login.keychain (in finder, cmd + shift + g, paste path, go)

It can be added to another machine and you can change the password of the keychain.