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system
what do brokerage firms do?
user
off the top of my head, a broker: while there are stock exchanges that offer direct market access (dma), they (nearly) always want a broker as well to back the first two points i made.  in that case the broker merely routes your orders directly to the exchange and acts as a custodian, but of course the details heavily depend on the exchange you're talking about. this might give you some insight: direct market access - london stock exchange
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