I think it could work out as long as you know what you are willing to accept. If somebody demands a discount from you, just tell them no. If somebody you up demanding that you drop everything to work on their problem right then, just let them know that you are concerned about their problem, when you except to try and deal with it, and quickly cut the conversation short.

As long as you deal with them in a way that is acceptable to you, you have lost almost nothing selling them a product once if they treat you badly. And you have gained some money, and as PG said if you treat them fairly and very nicely, they might change.