I don’t know of any shops or websites. There was a store in Jacksonville, FL many moons ago, The Kit Bunker, that had a great used section. He ran it out of a large garage next to his roofing business on his property and would buy/sell used/old kits. It was only open in the evenings and weekends.
You could try the Buy, Sell, Trade forum here. I have bought/sold/traded many kits there.
The prices are what the market bears. Ref this thread: