Dear …
I want to take this opportunity to say how impressed I was with the knowledge and the professionalism __(Name)__ demonstrated as __(he/she)__ presented __(his/her)__ __(type of)__ proposal to me. __(He/She)__ proved to be very helpful throughout the entire process, following up wherever necessary. I quite enjoyed dealing with __(him/her)__.
Regrettably, I chose not to make a purchase through __(Name of Company)__, however, I wish to point out, that my decision is in no way a reflection on __(Name’s)__ efforts. Quite the contrary actually. I believe __(he/she)__ is an outstanding representative of your company; someone who should be considered an asset to your organization.
I commend you on your choice of __(marketing rep/sales staff member/etc.)__. __(Name)__ represents __(Name of Organization)__ exceptionally well and is a pleasure to deal with.
Sincerely,