Most brokerages won't let you short a stock that's below $1...but they vary in their rules. Only an idiot would even think of shorting sub penny stocks with the need to put up $2.50 per share in reserve margin. That protects the brokerages, and helps people understand that putting up $25K to make, maybe, $100 is a poor idea. Even worse if you don't actually know which way to share price is going to swing.