The Moldovan economy is traditionally cash-driven, you may not always be able to pay by card (particularly outside of the capital city Chisinau). When paying by card, ensure that you maintain sight of it at all times.
The most widely accepted foreign currencies are the US Dollar and the Euro. You should carry some Euro cash. Notes should be in perfect condition or they may not be accepted. It’s not always easy to exchange Sterling for local currency.
ATMs in banks in Transnistria won’t accept cards from non-Transnistrian banks. The de facto currency in use in Transnistria is the Transnistrian ruble. It isn’t possible to exchange Moldovan lei or other currencies into rubles, or rubles into other currencies outside of Transnistria. Attempting to pay for purchases in Transnistria in currencies other than the ruble is considered an illegal act by the de facto authorities.