By shorting €100,000, the trader took in $115,000 for the short-sale. When the euro fell, and the trader covered their short, it cost the trader only $110,000 to repurchase the currency. The difference between the money received on the short-sale and the buy to cover is the profit. Had the euro strengthened versus the dollar, it would have resulted in a loss.
One of the most common trading and investment philosophies is to 'buy low and sell high' - this is particularly the case with long-term investments, such as investing in stocks or bonds, which rely on the asset increasing in value. In the Forex market, you can also sell high and buy low. This way, you can potentially make profits on both downward and upward trends.
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