"If you repair someones computer( you replace drive with new one) and then customer return beck with faulty drive, you are on very thiny ice.
It's unfortunate, but it happens (especially with Seagate drives or WD Blue drives). I try to always use WD Blacks as replacement drives; they are very reliable. My warranty is on the labour only, but I will usually give the customer a break in such cases, which are rare for me. I never install a used drive.