Of course, you will want to verify that the e-mail address is unique.
One method of registering custom validation rules is using rule objects.
To generate a new rule object, you may use the If you only need the functionality of a custom rule once throughout your application, you may use a Closure instead of a rule object.
Or, you may need two fields to have a given value only when another field is present.
Adding these validation rules doesn't have to be a pain.
For example, perhaps they run a game resale shop, or maybe they just enjoy collecting.