Cash them in.
Ignore the people who say there are legal issues here. There aren’t. You bought a used computer, as is, with whatever software and files were on it. It’s yours now. So is the copy of Photoshop, MS Office, or whatever else it came with.
This is essentially the same situation as if you buy a house or car and later find something hidden inside it. A man once found an original van Gogh painting in his attic. Another found a collection of comic books valued at $3,000. A California man once found a rare aluminum penny under his carpet - that penny was worth $200,000. Another found $150,000 in cash buried in his back yard. Countless other people have found rare coins, cash, and all sorts of valuable things in attics, crawl spaces, or hidden inside wall panels. One time a guy even found $20,000 cash tucked inside a book he purchased at a used book store.
In all cases, they became the legal owner of that property, by virtue of purchasing its container.
In your case, the computer is the container. You purchased it and its contents legally. The coins are yours. Just keep in mind that if you turn it into real currency, you will have to declare it as income and pay taxes on it. That’s your only legal obligation.