Covering is a simple 'buy' transaction. There are no margin requirements if you are not buying on margin. However, as was pointed out, since greys are not a marginable security, a short interest cannot be established.
Also, alot of people don't realize the ridiculous cost of shorting a penny stock. Basically, if you wanted to short 100,000 shares of stock that was trading at 0.01 you would need to deposit $250,000. But the most you could hope to realize is $1000. Now the chances of losing the entire $250k are small because the stock would need to go to $2.50 per share...but as you can see the downside is much more significant than the upside. Very few are going put that kind of money at risk for such a small return.
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