To refer to a real estate agent who is not a member of NAR as a Realtor is improper and misleading. It is analogous to calling any old jeans "Levi's" when they are not Levi's but another brand.
"Trademark infringement is the unauthorized use of a trademark or service mark on or in connection with goods and/or services in a manner that is likely to cause confusion, deception, or mistake about the source of the goods and/or services."
"The NAR has begun citing members for using the term “REALTOR®” to indicate a person’s occupation rather than their membership in the Association. If your marketing is similar to these examples, use “Real Estate Agent” instead of “REALTOR®” or you will also receive a notice from NAR. The REALTOR® marks serve the special function of identifying members of the National Association and should never be used interchangeably with terms such as: real estate agent; broker; salesperson. While HRRA encourages members to proudly display the REALTOR® logo on their marketing and branding pieces, members should make themselves knowledgable on issues of proper use."
http://www.hrra.com/_docs/media/trademark.pdf