Read the fine print on that guarantee. Most vendors require you to utilize a specific repair shop or service which may not be authorized by the manufacturer of that device. You only want Apple or an Apple Authorized Service Provider to repair a 2018+ Mac due to Apple pairing parts to one another which only Apple or AASP can do. Some repairs can be performed by an unauthorized repair shop or service, but you may lose some functionality of the Mac.
Also, be very careful buying a used Mac from unknown & untrustworthy sources. If the Mac is not properly prepared for resale, then the previous owner may still control & manage the Mac (not always obvious at first), or the Mac may have a firmware lock. None of these issues can be resolved except by the original owner of the Mac. Plus there are a lot of disreputable sellers online with some of them selling on Amazon. I highly recommend you only purchase a used computer from a reputable vendor (Amazon allows third parties to sell on their platform and these third parties are not always reputable).