Not USD/EUR, but EUR/USD(Making EUR as the first base currency) is the world standard when trading in foreign exchange market.

In fact, placing a “buy order” of EURUSD, is the same thing as placing a “sell order” of USDEUR.

Symbols like USD/EUR don’t exist, doesn’t mean that there is a limitation to the kinds of Forex currency pairs you can trade.

The Concept of Buy and Sell

If you expect the the value of USD is going to fall against EUR, then you may buy EUR/USD or sell USD/EUR(this symbol almost never exists though).

The first currency of a pair, is always the base, and a market price you see is the value of the first currency of a pair based on the second currency.

If a market price goes up, then the value of the first currency is rising against the second currency.

It is very difficult to understand the concept and the directions of values of currency pairs. If it is confusing, you are recommended to open a Demo trading account to practice and see them by using it.