Is it ever appropriate to ask a professor to be a business advisor? Myself and a few business partners of mine are working to develop a software product, in which I am responsible for building and all other technical aspects. At the same time, I'm currently pursuing a secondary degree in Computer Science at a local university. As my responsibilities within our business include securing a competent technical advisor, a suggestion was raised by my colleagues to inquire with my professors as to if they'd be willing to take on a limited advisory role in exchange for some amount of company ownership.

Is this at all common in the academic world? There are past professors in which I've excelled in their classes, had positive relationships and would consider their knowledge in such a role incredibly valuable. Additionally, I also know a few of them are not full-time with the university, but serve as guest lectures with ongoing careers in the software field.

I've been told this isn't an entirely uncommon practice, but I can also foresee a number of ethical issues and conflict of interest surrounding this idea, given that I'm a student within the program.
Due to the mentioned ethical aspects you should avoid any financial relationship with professors of which you are still taking courses. 

On the other hand, it is happening quite often that a professor is supporting a start up and compensation can be either money, company ownership, or sometimes just personal satisfaction.