In any industry, someone wants to their offering introduced to someone else.
Larger companies will have their own marketing and sales departments. But smaller, newer outfits may not have the skill set, the personnel, or the bandwidth to make as many connections as they'd like. So there's a surplus of opportunities out there for them but a deficit of time.
That's where you step in (potentially). Online lead generation requires at least a relatively small budget. But person-to-person legwork costs nothing. So any success you can bring about puts you in the black.
These opportunities exist EVERYWHERE. And brokerage can be scaled into bona fide lead generation or expanded to include marketing services, consulting, etc, etc, etc.
And you can always pivot from the broker role to something more rooted.
Acting as a liaison is also a great way to make professional connections and familiarize oneself with the inner workings of an industry. Later on, if you want to invest in setting up some cash-hungry venture, you'll know your way around -- which greatly increases your chance of success.
Almost any business can be, but those with limited start up and inventory costs obviously fare better, faster, but sometimes are more difficult to scale as others have suggested. I'd suggest reading Profit First by Mike Michalowicz (his blog and source for the book is at http://www.mikemichalowicz.com/money-strategies/profit-first-money-strategies/) for a way to switch thinking to achieve profitability and positive cash flow first in almost any business.