I think that depends on the integrity of the retailer.

I’ve recently made a number of N.O.S. purchases of discontinued eurorack modules (older intellijel stuff, mutable instruments) from Analogue Haven, Big City Music, and Detroit Modular. In all three cases what I purchased was definitely unused/unopened.

If a retailer advertised something as N.O.S. that was clearly a return or used item I’d demand a return/refund and they’d lose my business.