I've got a pair of DCs (H10-13.4) that are now some 16 years old. I suspect they're young compared to some. They are as good as when they were new. If you want someone to test your DCs prior to sale, then visit your local flying club - either ask them to test them or go for a flight in the righthand seat yourself if you no longer qualify for the one on the left. You'll probably find a buyer in the same place.
Returning to the thread, why would anyone want to build their own headset, particularly if it risked damage to an aircraft's expensive electrical system? New, they can be bought for little over £100 so why bother. They are also your main communication lifeline to the ground!