your initial question is a little long but, I do many top dressings but I also spray the top dressing is actually very easy to sell. In fact I try to stay away from it as much as possible because it is a lot of work. I spray, mow, mulch and do top dressings. If you set up a complete service like I have people are much more inclined to use you for everything. To "sell" a top dressing you need to let them know that it is one of if not the best thing they can do for their yard, you can fill in uneven spots it helps the lawn during drought conditions because you are making the root system longer. A customer cannot get all the results of a top dressing just from spraying, an educated customer is a good customer tell them all the benefits and you are good. I hope I answered your question.