I mailed out 3 books and didnt mark them 'sent' in time. I messaged all 3 and told them their books were in the mail and please mark them received when they got them and all 3 responded but one wasnt sure how to mark it received. I think you do it by going to transaction archive but after that I'm not sure. Could someone refresh my memory?

When they look in their TA, there will be a button next to all unsuccessful transactions that says "Was book Recieved?" and they can click on that.

They need to make sure that the request is not rolled over to another member. If it has, they should try to cancel it and if they can't cancel it and end up with two copies, they should not mark yours as recieved. The Help docs explain this. Verifying the sender is really important if two copies are on the way.