Peter, I think it somewhat boils down to your motivation. If you're thinking you want to start a new business similar to the one you sold solely because it can make you more money and "legally, it's ok," then sure, that's probably a bit unethical and rude.

But face it: by your own admission, you didn't really know what you were doing the first go around. You think you can do better this time, and, most importantly, build something that can scale better and be very useful to a wider range of people and enterprises. You'd be building on past experience to create something better that adds value to the world and (possibly) wouldn't exist otherwise.

Try to look beyond your own particular situation and think, "could the world be better if I started working on this stuff again?" And if you think the answer to that is yes, I feel like you should be able to proceed without any feelings of guilt or rudeness. The fact that you can make some money doing it is certainly motivation (we're human, after all), but it's almost besides the point.

You sold your company, and you honored the terms of that sale. If you think you have an opportunity to do something awesome that you'd enjoy and you're passionate about... go for it!