How Do We Handle Used Stamps?

How do we handle used stamps? I work in a town that is notorious for trying to reuse old used stamps ( that weren't marked) by cutting them out of old envelopes and gluing or taping them to new outgoing envelopes. So i cross them out and return the outgoing mail back into mailbox. This customer says that they were new stamps they cut out of envelopes that were never sent out. Therefore the stamps weren't "used". What would you do?