You can define your default currency in the Preferences Panel. If you intend to use only one currency in your future transactions, this is it. You do not have to bother about currency any more; all new stocks, all new transactions, all reports will be in the defined default currency.
You can also define the default currency in the Currencies management panel
If you intend to use multiple currencies, Mastock is extremely flexible. A different currency can be defined for each new stock. Further transactions, buy or sell, will take into account the currency defined for this stock.
Each transaction is, by default, using the default currency. But you can decide to affect a different currency for each transaction.