To be a good resume writer, you "must":
1. Be a good researcher / interviewer / listener
2. Have knowledge of journalism / writing principles.
3. Have "advertising" knowledge -- what makes people (employers) buy? What do I need to know to "sell" my clients to prospective employers?
4. Demonstrate Sales Skills / Customer Service Abilities. If you can't sell, you won't have anyone to write for. If you can't keep them happy, you won't have referrals or repeat business.
5. Utilize Time Management Skills. In order to make enough money to succeed, you need to write relatively fast and write relatively well. Being able to prioritize your time to manage your business (billing, invoicing, paying the bills), generate clients (advertising, marketing, referral relationship building), and write resumes is critical.