Let’s imagine a world where time machines are invented.
Hypothetically, what’s stopping anyone from travelling to the past, where the dollar is much more valuable, and buying things at a much lower price? What if you then go back to the present, sell those things at a higher price and repeat the cycle? And wtf would happen if everyone there started doing that?
I feel like just talking about the buying power of money or even the ability to effectively duplicate stuff is missing something: assuming your time travel actually allows you to change the past, and it doesn’t just end up in a situation where you can only fulfill a timeline that always existed, you can take technology to the past too.
Go back to the beginning of civilization and give them current technology (or even the beginning of time, and found a new civilization there). Then do it again (or if you don’t, someone else will eventually) with the new tech developed off the existing stuff over time. Repeated ad nauseum, you end up with a situation where civilization has and since the beginning has always had, every single technology it is physically possible to create.
You get the same kind of issues as “the singularity” (the concept where a super intelligent AI improves itself exponentially until it is as powerful as is possible to be). As such, our entire concept of markets and money and economy are likely completely obsolete, because find yourself in a universe populated by something as close as is physically possible to become to gods.