You can install it yourself but you should not be able to purchase 3rd party hardware with it pre-installed.
It you do I would strongly recommend you reinstall it to be sure what you have is actually unmodified pfSense.
If the license expires you will lose access to the pkg repos which means you can no longer see updates or install new packages. It does not prevent existing services running.
If this is used Netgate hardware it will always have access to Plus, that does not expire.
Steve