First of all, it is illegal in every state to sell a used product as new stock. That said, B-stock, returns, or factory refurbs get another round of quality testing before sale, and so actually have a lower failure rate than new components.
Some years ago I visited a production facility for PC Ethernet adapters. The entire factory required only 5 people to operate. QC consisted of conducting a POST (Power-On Self Test). Cards that completed POST were deemed good, the others were sent to junk, with no attempts to repair. It was explained to me that of the cards that would POST less than 0.1% would fail (1 in 1,000) within the first few hours of operation, after that less than 0.001% (1 in 100,000) would fail over the next 5 years, and after that, the failure rate would slowly drift up until about year 10, when the cards would be obsolete. Their MIL-Spec cards were identical, but got 48 Hrs of burn in, and cost 5-10x as much. For context, this facility made over a million cards per year. The point is almost all electronics share this same basic failure curve. After the first few hours or days, failures are very rare until you reach the end of their service life. Tubes are replaceable wear units (think tires and brakes), but also are still quite reliable after the initial burn-in.
That's why lots of us prefer refurb units. They may have had an early failure or were misconfigured, but as others have pointed out, received additional burn-in and testing, and so are more reliable than new, but cannot be sold as new, so are discounted.
