Realtor is a licensed trademark from The Association of Realtors. You can see the Trademark here:
Trademark Electronic Search System (TESS)
Just because something is popular in idiomatic expressions (xeroxing, rollerblading, googling) does not mean that the mark is not vigilantly protected by the parent organization.
If you don't comply, these guys will UDRP you. The National Association of Realtors is known for being vigilant about protecting it's mark; as is Geico (don't you dare use a Gecko), McDonalds, Disney, Coke, etc.
I'd comply and alter the domain to say 'real estate agent' or something similar. This is what you meant anyway, you're just calling the xerox machine a photocopier instead of a xerox this way.