Attorney Jeffrey P. Coleman explains when a broker may (or may not) buy or sell on your behalf.
When a broker buys something without your authorization, he is violating one of the most basic rules in the brokerage industry. It’s your money. If you have not authorized that broker (in writing) to trade on discretion in your account, he has violated one of the basic tenets of the industry.
They are paid to place transactions: your transactions. Be very careful about giving discretion to any broker to trade in your account.