Once Sony would no longer accept returns on those CDs, it only made sense for stores to sell them as used at a reduced price, ...

It's not only the condition of the CD itself that matters, but the external components too. If you sell a heavily used CD with a torn lyric book under the guise of being a mint condition CD ...

Once Sony would no longer accept returns on those CDs, it only made sense for stores to sell them as used at a reduced price, which, in turn, led to a willingness to buy used CDs from customers.