Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
I would like to let you know that I have decided to sell my [Business/Property/Practice]. After having my [Business/Property/Practice] for [#] years, i have developed solid and productive relationships with both clients and partners. After much thought, I've realized that the timing is right for me to move on.
I would like to introduce you to [name], who will be taking over my position effective [date]. Rest assured [he/she] is a very competent [businessman/woman/etc.] with [#] of years experience behind [him/her]. It is important to me to find a perfect fit for my clients and I feel confident that [name] will be very knowledgeable and a pleasure to work with.
I thank you for your business support over the years. It was my pleasure to have been of service to you and I wish you continued success in the years to come.