how to get started as AC broker?

we both want to remain connected to aviation and are thinking of opening
a small aircraft sales/brokerage business, focusing on basic airplanes
such as Cessna 172/182, Piper Warrior, Beech Bonanza, etc.
[...]
I refuse to believe that with the right mindset, business practices, and
a good plan, the playing field cannot let another newbie come into the
business.

Find a niche. Here's one I'd suggest - focus on putting people together who would make a good partnership in buying a plane, and then sell them the plane. There are many people who are beyond the FBO, not ready to buy a plane, and the other easy
option is a flying club. It's a good option but has its limits (such as performance and availability), and the next step towards ownership would be a partnership with (say) four or five people. However such a partnership is difficult to find, set
up, and manage, and there are "bad partner" issues that increase the more partners there are. If you made it your mission (and marketed yourself that way, and were good) there's a big opportunity for getting people one step closer to the freedom of
their own airplane.

I'd say that to make this work, you should have connections with A&Ps, and with consellors (of the type that could help determine who would make a good match with whom, and who could be called upon when a partnershp has rough edges). Also, as you
make more connections and create more partnerships, there will be more options for fixing one that goes sour, and a possible "upgrade" route for individual partners.

Where are you located?

Jose
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Note - Since the OP is crossposted to r.a.misc and r.a.owning (which I don't follow) as well as r.a.piloting (which I do), I posted the reply to all three groups. For me to see replies, include r.a.piloting in the list.