Don:
Consider this concept for your business. You may have 20-50 harnesses laying around waiting for a customer who wants one. They are taking up space, and your customer demand is probably no where near your inventory. Why not sell half of them to me, get a rather large lump sum, free up some space, and further help out the Mustang community by recycling an apparent low demand item to someone that can use them immediately? I've dealt with several small Mustang vendors just in this way, and they are so happy to get rid of stuff that they've hoarded for years for little return until I showed up.
I buy underdash harnesses for $50 regardless of condition and $35 for headlight harnesses. I also pay for your shipping charges (ground only). If you believe the harnesses are in very good to excellent condition, we can talk about prices.
What people think are harnesses beyond repair can be repaired once one has 3-4 for cannibalization to make one good one. To repair harnesses requires knowledge, adequate materials (connectors, correct wire colors, plugs, pins, etc.) to repair, adequate storage facilities, etc. Those with a single harness simply don't have the parts and connectors to repair most damage. The only harnesses not worth anything are those that have been in a fire or have had an electrical short that has damaged surrounding wires.