I’ve bought several used Canon lenses, including a 135 L from B&H a month ago. So far I’ve had no issues with used purchases, however I have stayed away from third-party lenses and Canon models repudiated to have QC issues (admittedly, none come to mind). Also, be aware that with some used items, the original packaging or user guides may be missing, but the listing will generally state when this is the case. Finally be sure to check the warranty length and inspect the lens within that period.