You can support buyers as long as Stripe, PayPal and your generic payment processor(s) (if applicable) have the ability to send payments in any currency they support for that particular country.
Take note that they may incur (1) Bank conversion fees and (2) Foreign transaction fees by the bank should they choose to pay in a currency that is not the marketplace’s default currency.
Example: your buyer may purchase using HKD even if the marketplace’s default currency is SGD. Stripe and/or PayPal will do the currency conversion and the charges will be borne by the buyer.
You can support sellers as long as Stripe, PayPal and your generic payment processor(s) (if applicable) have the ability to receive payments. If the seller chooses to receive payments in a currency different from that of the marketplace’s default currency, (1) Bank conversion fees and (2) Foreign transaction fees by the bank will be borne by the merchant.