No. You are not required to obtain a patent in order to sell a product or service embodying your invention. Many products and services are sold that are not patented.
A U.S. patent provides the right to stop others from making marketing, selling, or importing your invention in the United States. Therefore a patent may provide you a competitive edge against your competitors in the marketplace. But, a patent is not a grant of permission to make your product or provide your service. A patent will not protect you from claims of infringement by others.
For example, assume that your product is A+B and you obtain a patent on A+B. Then John who owned a prior patent on A sends you a letter alleging that your product AB infringes his patent A. You cannot use your patent on A+B as a defense. This is true because patents can be used offensively (to stop others from making, selling, etc. your invention) not defensively (to prevent others from claiming your product infringes their prior patent). However there may be other defenses that you could assert, such as that the patent on A is invalid. When faced with claims of infringement, always seek the advice of a patent attorney to assess your options.
In order to be sure that your product or service will not infringe another party’s prior patent, you need a clearance or freedom to operate opinion (to find out more about his option click here). In order to prevent others from copying and selling your invention you need a patent. However, there is no requirement that you obtain a patent in order to sell a product or provide service. You may choose to proceed without a patent and compete in the marketplace.