It doesn't make too much sense. Put yourself in the shoes of the shopkeeper at this moment. You happen to have scrounged up some decent weapons, maybe some special ammo, but you are clearly no man of action. You site in your barricaded shelter and hope everything on the outside turns out alright. Things aren't, though, and the future looks bleak. In walks our hero/ine - just the person to get the job done. This is someone with a proven track record of saving the day, of helping those in need, and being an all-around pretty cool cat. He needs better equipment to face the current danger threatening your town. Now, make your decision:
1) You be a dick. Oh, you want that magical sword that would certainly help you save us all? Sure, if you have a 20,000 units of whatever bullshit we're using for money that I won't be able to spend because if you fail we all die. Don't have it on you? Well get your ass out there, face things that may kill you, and in turn kill us, and get more shiny things!Now comes the video game aspect to it. Sure, if everyone just gave you everything you needed, video games would become way too easy. So how do we make up for that? How about instead of accumulating money during these situations you, instead, accumulate favor points. Saved an old man from certain doom? 25fp. Killed the four demons that threatened the city? 5,000fp. Made a poor kid a sandwich? 5fp.
2) Assist. You need this magic sword? I will happily give/lend this out to you. You are an incredible person for manning up and saving the world. Few would do so, as would be the case as you are the only one. Go forth, young adventurer, and bring peace to the world.
"Now wait a moment, Eisbaer," I hear you question in my head, as your inquiries are imaginary, "doesn't this just directly replace money for a different type of currency?" Some yes, some no. Just because you earn so many favor points and that blacksmith finally gives you that chainmail suit you've been drooling over it doesn't make you less of a good person, so you don't deduct your points from the total.
However, I also feel that you should be able to offer favors as well. 'Look, I realize this blessed blunderbuss has been in your family for seven generations, but I really need to use it to kill this monster.' Seems reasonable, right? So this would be a moment in which you would incur favor debt. You now owe someone favors. Maybe once the monster is defeated they're going to need you to run some errands for them. Easy enough. You could have done it before hand, but darn if those life-giving herbs weren't on the other side of the monster's den. So now you have to remember to pay off that debt. "But why should I," the self-important hero asks my brain, "I've already got the blunderbuss, I've got world saving to do." Because making empty promises makes you a bad person and that debt starts hurting you. Chainmail and blunderbuss acquired, sure, but the CEO of the evil corporation that is polluting the air and water is protected from everything except a rare breed of hamster. Well, the dude that sells those hamsters learned how you stiffed that guy, and seven generations of his family, over that blunderbuss and heard that your head smells. As such, you will not be getting any hamsters from him unless you make up for it. This would either be in the form of collecting far more favor points or going back to the House of Blunderbuss, apologizing profusely, and doing that favor you promised to do.
So that is my recommendation to replace currency systems in settings were money just doesn't make sense.