I’ve been flipping cars for profit for over 10 years, and it’s been a great source of supplemental income.

That way you can use your current income to supplement your investing income and achieve your goals faster.

I'm an IT professional looking to supplement my income and eventually exit the mandatory workforce.

Then use a mailing list to supplement these. 

Fast forward a some years and now I'm making a good living doing that for a supplement company.

My long term goal is to supplement our retirement income in 20 years.

For example, taking 100 acres and making a city, how cool would that be.The good thing about REI is, its flexible and you can do it as a supplement to whatever your doing now or plan to do.I say do both, maybe use your RE license to fund your education in architecture.I agree wholeheartedly with @Ryan Koehler , especially if one is independent.

(I considered it good practice for screening future tenants.)If you're seeing cheap room rent in places like Oakland, Berkeley, or San Jose, that could be from A) larger houses with lots of rooms, B) older homeowners looking to supplement income, or C) homeowners looking to attract college students or startup employees. 

I'm more interested in buying and holding small multifamilies to supplement my current income stream (I work in road construction).

My goal is to build enough residual income off of real estate in order to supplement the $3,000 a month I'll get from my disability check every month.