I certainly think that if you consider your time spent a "sunk cost" because you are going to build anyway, that is no longer part of the equation, and sure, you can profit on materials. To me, I would personally see it more as a way to "offset" my costs in the hobby at that point. As it stands already, I do not think any amount of selling, either unbuilt kits, or finished kits, can cover the cost of what I have sitting in my hobby room. Between suppliers, paints, tools, kits, it would be hard to quantify the overall investment.
That being said, I really only want to build what appeals to me personally any way. Either in general, and at any given moment in time. Time spent building a subject I have no interest in would start to feel more like work to me, and step away from why I personally build. Much like my wife asking me to paint a room, because I like to paint models. NOT the same thing. Over the years, I have given several models to people, and built some FOR them, but I still have to have interest in the subject matter.