I wouldn't offer more than $3500 even if sealed , warranty transfer is costly with the mandatory purchase of new tubes
I just don't see the value in buying a used Decware. Part of its allure is the Lifetime Warranty. To obtain a warranty transfer is going to be almost $500, not to mention tubes which there are many of in that amp.
The UFO84 is worth sending in to transfer warranty because it's $150 or less and the tubes are cheap.... more expensive Decwares have a number of tubes that add to the cost. Good luck