I think you are missing the original point of the discussion. It's not about what the developer chooses to use, it's how the client perceives the cost of the solution.
If the client is given the perception that a full blown shopping cart with a list of bullet point features should sell for x dollars, my looking at it as just another tool is not going to have any effect on that. The expected price point has been set, regardless of my mentioning it or not. They are not going to be thinking about how well it will scale, or if key options are missing.
If someone makes the decision to purchase a ready-made solution directly from the publisher, instead of from you, it's not likely that you would still consider them as your partner as you receive no benefit. The companies that I've "partnered" with will not sell to end users. This removes any possibility of them becoming competitors. I'm only competing with their other partners, not them directly.
If there are no competitors in you scenario, it means that you are in such a specialized niche that you can charge whatever you like and no one else can compete. Not all of us are so fortunate.