Adam bought an iMac from a private seller. It still had Apple Care and he had it transferred to his name. He's worried that there was a keylogger on it and his credit card was compromised. Leo says that unless he wiped the computer himself, he won't know if it's compromised or not. Leo says that it's probably not the Mac, but just in case, Adam should wipe the drive himself. It's really easy to wipe an iMac drive and reinstall the OS. It could be that Adam's iCloud account has been compromised. Adam will also want to be sure the Mac has been removed from Find My Mac and that the iCloud account of the previous owner has removed it. Then he can wipe the drive and reinstall the OS.
There's probably a skimmer at a place where Adam shops and he's not even aware that it's capturing his card. PCMag.com has an article about how to spot and avoid them.