Assemble the purchase trail
Purchase orders, packing lists and submittal sheets tie a reel to a specification. Even a partial trail raises the basis, because it confirms the construction and shows the cable was never installed.
Photograph the untouched condition
Original banding, factory lagging, sealed ends and an intact tag all say 'never installed' more clearly than any description. Photograph them before anything is opened or moved.
State the release position
If the cable was bought for a client project, confirm who owns it now. A short letter or email confirming the material is free to sell avoids a dispute later and is expected on plant and utility work.
Keep the lot together
Cancelled-project cable is usually a coherent set of constructions bought for one specification. Selling it as one lot preserves that story and usually beats splitting it into unrelated pallets.