15c3-3 does not say that a broker can not use client funds to purchase securities for the client that the client is trying to buy.

It says the broker can’t use client funds to purchase securities for proprietary trading.

Everyone has totally misread the rule and then is stating a total falsehood as if it’s unequivocal truth. It’s maddening.

> The Commission adopted Rule 15c3-3 in 1972, in part, to ensure that a broker-dealer in possession of customers' funds either deployed those funds “in safe areas of the broker-dealer's business related to servicing its customers” or, if not deployed in such areas, deposited the funds in a reserve bank account to prevent commingling of customer and firm funds.[7] Rule 15c3-3 requires a broker-dealer to calculate what amount, if any, it must deposit on behalf of customers in the reserve bank account, entitled “Special Reserve Bank Account for the Exclusive Benefit of Customers” (“Reserve Bank Account”), under the formula set forth in Rule 15c3-3a (“Reserve Formula”).[8] Generally, the Reserve Formula requires a broker-dealer to calculate any amounts it owes its customers and the amount of funds generated through the use of customer securities, called credits, and compare this amount to any amounts its customers owe it, called debits.[9] If credits exceed customer debits, the broker-dealer must deposit that net amount in the Reserve Bank Account.[10]

https://www.federalregister.gov/documents/2004/09/07/04-2018...

> Possession and Control Monitoring is related to SEC rule 15c3-3 - The Customer Protection Rule. The Customer Protection Rule prohibits a brokerage firm from commingling fully paid for customer securities with non-fully paid for customer securities and/or firm securities.

> When a client of the firm purchases securities through the firm and pays for those securities in full, the firm must "lock-up" or "segregate" those securities for the protection of the client. The segregation of fully paid for customer securities protects those client owned securities from the firm's creditors in the event that the firm can no longer continue as a going concern - that is, the firm goes belly up.

> SEC rule 15c3-3 also prevents the brokerage firm from using full paid for segregated shares to satisfy any delivery commitments that might result from the firm's trading activities. If, for whatever reason, the firm does use segregated customer shares to satisfy a firm delivery a 'SEG Violation' or Possession and Control violation occurs.

http://brokerage101.com/opscntl.html

> The reserve formula calculation is performed weekly to identify customer related payables and receivables. If the calculation results in a net payable to customer, the broker-dealer is required to reserve this amount in a special reserve bank account. This special reserve bank account is for the exclusive benefit of customers and funds can only be withdrawn with the support of an updated reserve formula calculation. In theory, on the date the reserve formula is calculated, combining the securities held in possession or control plus the funds included in the reserve formula should equal the amount the broker- dealer owes to the customer.

https://www2.deloitte.com/content/dam/Deloitte/global/Docume...

> Rule 15c3-3 prohibits use of customer property to support non-customer activities.

https://www.sec.gov/rules/final/34-50295.htm