Ameritrade and Scottrade do not allow trading of ELTP on a margin account. The only thing a margin account does under the circumstances allows traders to buy and trade without regard to funds clearance. My best take on it.
No, my account doesn't allow borrowing on penny stocks and certain other very volatile stocks but it does allowing borrowing on the more "blue chip" type stocks. I know I can't borrow on my ELTP shares (even though I have an account that permits me to have margin) but I was trying to avoid having my broker lend those shares out to shorts. Just as I don't use stop-losses I was trying to prevent shorts from being able to borrow my ELTP shares.
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