With no offence intended to you, that site is far from accurate and in my opinion it under values things, in fact they would offer about $475 for that particular machine...
The most accurate way of deciding the value of your computer on the secondhand market is to search ebay for the same model, select completed listings and then take an average price from those that have sold in the last month, then you know what people are willing to pay and that will tell you the market value of your computer.
No other way is more accurate than that, because it's only worth what people are willing to pay.
A quick scan through eBay would say that about $750-850 would be about right for that machine and more likely to get the top end with Apple Care and it's original packaging.