I'm not a broker, however it happens that I do trade, but only for my personal account.
About a month ago I bought a "something."
The other day I looked, liked the price, and entered a limit order to sell.
[That's at a particular price.]
I nailed it!
The trade went off three minutes before the market closed, and I got the best price of the day.
I was feeling good.
Now there was a little bit of "extra" available.
I was trying not to feel smug.
That lasted until about two hours after I got home.
... and then the washing machine broke.
Mr. Murphy may be in his grave.
[There really was a Murphy, and he died around 1991.]
But his law lives on.