This is a thing for pets - it should be a thing for items too I believe!
If you trade away a pet in a different trade, the older ones are cancelled with the message "(This trade was automatically cancelled because one of the pets in the trade was moved or deleted.)"
I am a firm believer of item-rights so I feel like items should not be disregarded in this! Equal rights for items! Items are pets too!
But in all seriousness, it would make my life a little bit easier, and I'm sure it would as well for other people who trade a lot of items.