Hi there!
Sizing can vary considerably between manufacturers so I’d always recommend consulting a professional (or a geometry chart for that specific model, if you know your measurements.) My road bike is a 52, but my Canyon TT bike is a 48.
I can’t say any particularly awful models spring to mind, but standard caveats apply- fit comes first, check the wheels are true and there’s no faults in the carbon (might pay to bring a friend if you’re not sure what to look for), and get it checked out by a bike shop after purchase- though preferably the prior owner has done that for you already.