The resell is to different customer right? The original trade-in from customer A provides customer A a reduced price on another item that they are purchasing. The trade-in item after refurbishing though could be sold to customer B, C, etc., right? Enter the trade in as a negative qty on the order/invoice using a separate warehouse code to distinguish the trade-in yet to be refurbished. Enter no cost, just a price which will discount the sale of the other items on the order/invoice. Create a new item using BoM or WO, to track trade-in, additional parts, labor and use the refurbishing warehouse code for trade-in component. The parts, labor, etc. become the 'cost' of the refurbished item.