There is actually a group that publishes a "Blue Book price guide", but you would really need to know someone in the audio resale industry to really get current info.
Unless the amp is truly a memorable/desired name- such as McIntosh, most electronics older than about 3 years, sell at garage sale prices most of the time.
Realistic range for resale to a private party is perhaps 20 to 50 dollars. This is just a "from the hip" range. Pawn shops will be reselling anything they get their hands on, for well above what the real market is.
Not the answer you were hoping for, I am sure.
: Anyone familiar with the Pioneer SX1280?
: Worth anything?