No. If you have received your order in the mail, or your order has been prepared to ship, you cannot sell your tickets back into STEP. Shipping will begin in mid-June. Only tickets that have not shipped or are set for pick up at Will Call can be resold through STEP. If you want to have the option to sell your tickets through STEP after fulfillment begins, you can email participantsupport@burningman.org to change your delivery method to Will Call; however, this change can only be made before your order has shipped.
You are, however, welcome to sell your tickets outside of STEP for face value plus fees once you have received them in the mail.