I'm trying to sell an almost brand new Hypex Nilai500 with no luck. The resale value for those class D amp is pretty bad. Maybe a Nad with brand name recognition will give a bit more of it, but I wouldn't expect much.
You are selling a DIY product at full retail price, that is always going to be a tough sell. Certainly not comparable to trying to sell a commercial offering at a reasonably reduced price a few years down the line.
Michael