check the A'gon Bluebook. It will tell you what they have sold for. You can also check ebay - do a search on auctions that have completed sold.
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The basic rule of thumb for used equipment is to price it around %50 of the original retail price. Since you know that you spent $1200 to $1500 you could ask about $700 and see if you get a buyer. If you think it is worth more to start negotiation then price it higher. The price you will get is only the amount the market will pay.
I don't think the unused channels waste a lot of power but I am not certain. Usually you can leave an SS amp on idle and in most cases it uses a low baseline amt of energy.