Vendors can select their own display currency in
Garnet > Settings > Currency. By default, the display currency is the marketplace's defaut currency.
Every time a currency is displayed, it will use the day's conversion rate. You can hover on the currency to see the actual value the marketplace uses.
Other price inputs will properly use the display currency and do the necessary conversion.
Exchange rates are taken from the European Central Bank. The rates are updated daily at around 16:15 CET.