This is one of those things where it's basically impossible to make a game with more accurate economics that is actually fun. The fact of the matter is that medieval wealth inequality was just too big for most people to wrap their heads around and would make gameplay really weird. Adventurers would in fact need to be buying equipment with the equivalent value of gold coins, but such wealth would dwarf the costs of pretty much any "normal" stuff you could buy and would cause weird balance issue. For example, a pound of cheese in medieval England cost half a penny, but a good sword could cost 480 pennies. Think about how many swords you encounter in a video game. Even if you sold them for a 100th of the high end price, you could still buy 9 lbs of cheese for a single sword and if cheese is meant to be a healing item then it probably has to be total trash to balance how cheap it is compared to adventuring gear. Or you could say a low quality sword can be sold for 5 gold and a cheese is 1 gold and make it a normal healing item. It's just hard to balance if the economy is realistic. As for credits, it's just hard to imagine what the hell trade will look like in the future and everyone kind of understands credits as a concept.
