TLDR
- @Craig's answer is correct...
- The OASIS credit is not really an in game credit.
- The OASIS credit is more like Microsoft Points or Bitcoin
- People not only play games in the OASIS, but also work, shop, and more
- The book explains better than the film... of course...
I am just going to try and take a stab at explaining this situation better than @Craig's answer. His answer is correct, but I do not think you are understanding the situation as he describes it.
So the OASIS credit is not really an "in game currency" as you understand it. Remember, within the OASIS you live, work, play on multiple planets, platforms, situations, etc. The simulation is huge. Consider the OASIS at large like the Microsoft store at the hub of Xbox and all the things you can do within the OASIS (work, pvp, use internet, shop, etc.) as the games and apps you have installed on your console.
Back in the day Microsoft used Points to pay for things on the console. You would pay $50 and it would be exchanged for 50MPs (50 Microsoft Points). Then you would use those points to pay for new games, character skins, etc. You didn't use those points in the games, but you could use them to purchase items or dlc for those games.
Another way to think about it that might work better is Bitcoin. Hopefully you understand a little bit about cryptocurrency. Though Bitcoin is not widely used it is a currency that exists. If I convert all my money into Bitcoin, but a seller does not accept it that does not make my Bitcoin any less valuable, it just means I need to go somewhere else to use it. Kind of like how sometimes business do not accept certain credit cards, PayPal, or even foreign money. I can't pay with yen in a US Walmart. Does that mean any yen I have are worthless? No, just that I need to take it somewhere it is accepted.
In the world of the RPO universe, the currencies of the nations of the world are not trustworthy due to wars, famines, corruption, etc. In addition, due to the popularity and success of the OASIS businesses have evolved to allow people to work from home (across the planet even) and shop online from their VR rigs.
With that evolution people have also been offered the chance to be paid in OASIS credit. Rather than being paid by direct deposit of check to your bank account the business exchanges your paycheck within the OASIS to the OASIS credit and sends it to your OASIS account. Boom, transaction completed. All your money is now OASIS credit. If you needed cash or whatever you can still go to a bank, but most people leave it as OASIS credit...
So when you go shop now you are shopping online with OASIS credit. When you pay, you pay with it as well, and the seller accepts it. They still ship the product to you, but they accept your payment as OASIS credit rather than some other currency. After your credit is accepted it is simply transferred to to the employee who shipped it to you as their paycheck. Then they use it to order bread online and the cycle continues...
Hopefully that made more sense...