A good designer will understand the Internet mediums design options, both creatively and technically. It is NOT enough to just put up a site for you. There are issues of navigation, download time, technical implementation, sales process, marketing and your budget to consider and evaluate.
A good designer will proof for multi-browser capability, work to optimize your pages for the search engines, ensure functional, effective, and user-friendly navigation, and even address disability access issues or foreign language issues. And the nice thing about the web is that you can start with basic information and scale up to more advanced services as your goals and budget allow.
You might incur less cost up front, working with the teenager next door, but you have to ask yourself if they will be evaluating the issues related to your specific business? Your business deserves the best web site development practices, that take into consideration the unique needs of your customers and your priorities in business.