I prefer that the sender let me know in advance if it is an ex-library disc or an ex-rental. Or even if the movie is in poor condition. Then I can decide if I want it or not. I received one library disc from a swap member that I mailed back to the library. It did not look like a discard and I was fairly irate upon receiving it.
Since you bought your dvd's from the library itself, you should have no qualms about posting them. Just give the requester a head's up so they can decide if they still want them.