As a starting point, you can check
Mac2Sell,
eBay (including completed sales), and
Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc. You can also check the Apple refurbished store to see what similar models are selling for.
Don't check just one source, but get an average from all sources to get a realistic pricing idea. For example, Mac2Sell may quote a higher value than the market bears, while some sources may quote lower.
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