I buy and sell on Ebay quite a bit, and I include shipping on all of my sales. I let the market dictate the price of the item, as I price my items according to what others are selling for, and my customers are generally happy with their purchases. I did have an instance where a fellow wanted to buy a resin Transkit from me. He got quite indignant with me when I explained to him that the cost of a resin transkit was far greater than the cost of a comparable styrene kit, or die cast of the same item, and to add insult to injury, I was offering the kit for less than the manufacturer was offering it on their website. I steered him to that website and suggested that he buy from them. Never the less, I sold the transkit for more than the beginning bid anyway. I would have rather kept the kit, but I have one of the diecasts, and I knew that I would not build it in the near future.
All I can say is if you feel a price for that kit or part is to high, just go find one that is in your price range, if there is one out there.