I'm the buyer on that one. Unfortunately, right after I paid, they messaged me, and said that the keyboard doesn't work and that they were going to refund my money...
Too bad. Someone must've messaged them—probably just out of spite—and told them it was worth much more than they sold it for.
A completed eBay purchase is a legal contract; sellers shouldn't be able to squirm out of it that easily. But according to
eBay's page about it, they can; all they have to do is decide to do it, and refund the buyer's money.
Interestingly, the page also says:
If you cancel a transaction, the buyer can still leave you negative or neutral Feedback about the transaction.
I see this buyer has sold many items, but this is the first thing they've sold in more than a year. If you give them negative feedback now, they'll have a 100% negative rating! That's probably not something they want... Have you considered contacting them and pointing that out?
Otherwise, I'd take some screenshots of the listing where the label details are visible, then see if they relist the keyboard. If they do, it'll undoubtedly be for considerably more than you paid. At that point, I'd call eBay and complain that the seller is being unscrupulous. They'll have the whole thing on file and will be able to see what's going on. I don't know if it'll make any difference, but I wouldn't let it go so easily.