Somewhere around $350 - $400 is probably typical sales price for a working good condition set. The $500 ones have to be pretty pristine, and I'd want to know the seller and try the headset before plopping down a premium price for a used set.. The issue is probably not degradation of the electronics (the headband and ear pads are not very $$ to replace) as much as how much has headset technology improved since the 7 year old head set was manufactured with a 12 or 14 year old design.
Personally, I'd buy another QT Halo before I bought a 7 year old Bose, not because there is anything wrong with a 7 year old head set, but I find the Halos much more comfortable. The difference in cost would buy 23 gallons of 100LL, about a quarter of the Mooney's full fuel capacity of 117 G. Sigh. It's sad to think I could be carrying around $700 worth of fuel. I am flying less because of the fuel cost. Sorry, I digress.