Depending on your location, (USA, Europe, ETC) paying for freight on a large metal panel makes the deal cost prohibitive in most instances. If a buyer can order a part from a local dealer or specialist and avoid shipping charges and get a guarantee/and or inspect the part before final purchase, this would probably make more sense (if the ultimate cost was about the same.) If you have a local buyer, I would assume you could get 75%+ of retail value. If items must be shipped, the buyer will want to pay about 50% of retail value to offset the cost of freight. Cost savings of no tax/VAT on a private sale should also be considered when negotiating a deal.
Good luck and welcome to the FAQ. Please post your parts/prices and detailed photographs in the for sale forum when ready to sell.
Ben