Even if you had used it for the entire 15 years, the caps would be tired and best replaced, but probably still serviceable. But the symptoms you are describing are the result of caps in storage too long and they cannot be reformed by simply powering up the amp.
Even though there are only 25 or so total caps in that model, total replacement would be expensive because of the disassembly, parts and labor involved. I do not think it is worth it unless you can do it yourself.
I'm sure you could sell it as is on ebay or audiogon.
"If you can hear it, it's real"
Tom