Your marketplace can only transact in 1 currency (whichever you elected for your base currency), however you can display additional currencies for the customers to easily convert the amount into something more local for them.
In the backend as an admin, go to Stores > Settings > Configurations > General > Currency Setup.
In the field Allowed Currencies, select the different currencies you would like (hold CTRL + click for windows, CMD + click for mac).
Once done, select Save Config at the top right.
Once you have added multiple currencies in your configuration, go to Stores > Currencies > Currency Rates.
Here you can set manual rates for the base currency, from the different Allowed Currencies currently enabled.
Choose Save Currency Rates once you have the values.
NOTE: Choosing "Options" will take you back to the "Currency Setup" page in Configurations
NOTE: You can import rates and schedule updated imports using Fixer.io, however depending on the base currency you need, you may have to get the Basic Plan.
Subscribe to a subscription plan. See pricing here.
(Note that the free plan doesn't allow all base currencies - eg, AUD. You may need the Basic Plan for it).
Once you have an account, copy the API Access Key.
Log in as an admin in Omnyfy, and go to Stores > Settings > Configurations > General > Currency Setup.
Scroll down to configure the API Key and the scheduled import settings.
Fixer.io settings
API Key - Paste your API key from Fixer.io
Connection Timeout in Seconds - Set how many seconds you want to set before the connection times out.
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Scheduled Import Settings
Enabled - Set this to Yes if you are using an automated import of rates
Service - Select Fixer.io (or if you have another service installed, you can select that)
Start Time - Schedule when to start the import based on Frequency
Frequency - Schedule how often the currency import will run
Error Email fields - These are used for notifications on a failed import.
Once done, select Save Config at the top right.
Go to Stores > Currencies > Currency Rates and import your new currencies.
Choose Save Currency Rates once you have the values.