I am having one of my gas furnaces replaced later this week since I remodeled this past spring - It works just fine, but has some light corrosion on the interior from a previous a/c unit leak. Perfect operational condition though. The model is Armstrong Ultra SX 90. I just had it tuned up, its still at 89% efficiency and heating my home as we speak. Would this furnace be worth anything to anybody? Is it worth trying to sell this thing?? If so, how much should I ask for it? Thanks in advance!