I've tried to short it numerous times, with TD Ameritrade and also with Bank of America (Merrill Lynch). My orders have all been rejected. I would think that shorting zsl should be a pretty good long term bet, even if the trend in silver is down. Over time the price of zsl ought to be forced down, because if the fund managers have to rebalance every day, they have to continually buy low and sell high.
With TD Ameritrade....are you using 100% of your own money or are you attempting to borrow?
I've shorted zsl multiple times with TD and have had no issues at all. Of course you get the message that zsl has special margin provisions (since it's a 2x etf). But, as long as you're using your own money or not trying to borrow more than 30% on the trade...you should be fine.