If your exchange has no margin trading option, to trade BTCUSD on exchange you need to have balance in BTC and in USD, to trade ETHUSD: you need to have balance in ETH and USD, etc.
You can deposit on any currency (some exchanges allow to deposit only in crypto currencies, but some exchanges allow to deposit in your local currency like USD or EUR), and the exchange inside platform. For example, you wold like to trade LTCUSD and you made deposit to exchange in BTC. In this case you need to buy USD for BTC and LTC for BTC.
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