After your trade-in has been submitted, it can't be changed. You can either cancel and re-submit another trade-in, or only send the items you wish to trade in. The missing items will be marked as "missing", and you'll be paid for the items received if they're accepted after the inspection.
Note: If you've already posted your trade-in but changed your mind, please visit Your Trade-In Account and click Cancel. If your trade-in submission hasn't been processed before the cancellation was requested, your trade-in will be returned to you and no Gift Card will be issued.