If you designed the business knowing that it would collapse at some point in the future, and knowingly sold it to someone that would be left holding the bag, I think that would make you a very unethical person. If you did this, real people would be hurt by your decisions, and you would have continued to execute your plan knowing this. Maybe you just haven't thought through the consequences, but I wouldn't want to be friends with someone that acted that way.
Edit: Since it appears that I wasn't clear enough, I'm saying that misrepresenting the value of a business to employees, customers, and investors is unethical. If everyone involved knows the venture is doomed to fail and you can still retire off the profits, I don't see a problem with that.