Honestly, Toady wouldn't sell out, because Microsoft isn't that dumb. Dwarf Fortress is great, but the code is probably in such an unmanageable state that it would devalue it greatly. Lets be honest, its not worth a lot to begin with, because the player base is comparatively small. Barring some miracle, Microsoft either wouldn't want it, or would offer too little to temp Toady, who would require quite a large sum to depart from DF.
Everyone does have a selling point. Some people may be so detached to make that point insanely high, and in theory someone might exist that has no selling point but I doubt it. Considering Toady's lifestyle, he seems pretty detached from material possession, relationships (which imply children and inheritance), and he seems okay with his finances the way they are. I doubt he has much of a need for money, and does this for the joy of it, and that's admirable and even envious. So his selling point is likely insanely high.
GavJ has a good point, it makes business sense to sell if the price is right. Toady has gained a lot of knowledge about how to make a game like this, and he could redo things better, maybe manage things faster even. With his money he could pay to have a like-minded team help him, or even just a consultant to advise and help organize things. He would have time, and with the money it could even be done more efficiently. He could pay to have people document his code, do the menial tasks, maybe even instill good coding practices which I'm sure he doesn't follow. And no matter how he wants things done, he could always find a way for money to help. So if he wanted, he could redo things better this time around.
No, more likely a situation is that Microsoft decided to expand minecraft to have advanced features already in dwarf fortress. They wouldn't buy DF though, they would at best hire Toady as a consultant. He would probably decline unless the money was quite good, as it would just be a distraction to him.