4 - Can anyone explain to me what Unitrade means by [i]"Reply cards are considered used when the message portion is used and the attached reply portion is used.Attached cards with both sections used are very rare"
These cards have two portions: the original message card, and the reply card. Usually the card is perforated between, so the two parts can be readily separated. The original recipient received both parts with the "reply" portion still uncancelled, wrote their reply on the reply card, and then (most frequently) would detach the original message and mail only the reply card.
Occasionally, the replier would mail the reply card with the original used message card still attached. It was unusual for the reply card to arrive at its destination used, with the used original message card still present. Such cards (both halves used) are scarce and usually trade at a premium over either of the detached cards. (They often trade at a premium to the unused, in fact.)