Most retail shoppers are happy to pay about 1/2 of the original price for a used piece, and they ALL want it to be in perfect shape. Condition and demand will alter that from region to region. If a New "XYZ" listed at $1000 and was sold for a 30% discount($700)...an acceptable starting price would be in the $500 range. Dealer trade value is more like 25% of original list (subject to condition, marketability and availability)
There's no real science to it...it depends on the market more than anything. The original DX7 was PREsold at prices ABOVE dealer list because of all us Rhodes' users with tired backs and scratched up station wagons. It sold at list for about 2 years before the next wave hit. (M1, D50 etc..) Today, you can get one for $200 bucks.
Check eBay for the model in question and see what others are bidding. That's a good starting point, but remember - service, support and trade-ins are also factored into the price. It's not all dollars and cents. A good, honest dealer can be your best friend at times.
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