Brokers, meaning either the sales rep or the brokerage firm do not gain anything from their clients loosing money.
Yes I have heard of, and witnessed, sales reps doing this, but I worked for a regulator, specializing in fraud and bankruptcies. Such actions caused the rep to loose their license and the firm to be fined.
Neither the Reps nor the firms are permitted to play God in handling their clients transactions. Unless a clients signs a letter of discretion, the firm must follow the clients instructions, good bad or different as long as following such instructions do not violate any rules and regulations.
Reps and/or firms can be severely disciplined if they fail to follow their client's instruction, regardless if the client looses or makes money
Brokers make money by buying and selling securities. However, many brokers work as so-called investment advisers too, and make money as a percentage of their clients' portfolio balances.
Therefore, a broker has nothing to gain by placing a losing order, unless of course if he/she is personally at the other end of the trade. But that would unethical to say the least (perhaps even illegal), and they may be subject to dismissal.
I have not heard of it. That does not mean that it has not happened. These days with most retail customers trading over the internet where they get instantaneous order execution, I suspect that this is not much of a problem for most people. I also doubt that most brokers would do this. Why should they? What brokers are more guilty of is peddling particular stocks to retail customers that their firm wishes to unload. That is a problem.
Brokers are not there to help you make money, they are there to make money for themselves. Sad but true. It is entirely up to you to keep a close eye on your portfolio.
Can they do funny things when it profits them or their buddies? Hmmm... They stand too close to way too much money to be 100% uncorrupted!
A broker takes a fee everytime you buy and sell so if he manages to get you to sell prematurely (and then buy something else) he charges you multiple fees. Panic in the general population serves them well.
Can they manipulate the value of certain stocks by waiting? Possible. As small investors, we don't get the big picture of what is really going on.
Brokers get paid a commission on the trade. Most brokers are honest and work for the client. They don't want to provide bad service or their clients won't do business with them in the future.
Nobody uses market orders anymore. (Always use limit orders)
No. But that's why you should use limit orders - whether buying or selling.