Well...I believe the rules are such that they shouldn't be borrowing shares from cash accounts.....so it could depend on whether we believe the MM's follow the rules.
I also trade cash, but to be sure I don't leave anything sitting without an order, even if it's at 10x the current price.
And I've seen some highly suspicious activity before that led me to believe my shares were not where they should be, especially when I was trading heavy volumes on some shady stocks in the late 90's. I'm still using Etrade, same as then, and had some pretty sketchy discussions on the phone with some lame answers when I questioned activity.
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