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I don’t think you are going to be any safer using a used MacBook than your own computer if you are using a Ledger.
You should be just fine using your Ledger on your own computer. Even if there is malware on your computer, it is very unlikely that it is targeting your Ledger.
Instead of buying a new (used) MacBook, I would suggest just installing a good anti-virus if you haven’t already.
But a Ledger is designed to protect against malware on your PC, so it is very very hard (possibly impossible) for any malware to successfully attack your wallet, so you should be just fine with your own computer.
Personlly, I wouldn’t waste my money buying a used MacBook for this