Short selling is totally legal and although some people say it is "Un-american to bet against a company," it is actually something that keeps the market liquid. There are actually mutual funds that long-short stocks. There are ETF's that are only for short sales - like DZZ that shorts gold. Offshore short accounts are used outside the US because they aren't held to the same SEC regulations. The legality of what they do and how people get around this I don't really know about.
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