Arguing about the accuracy of the calculated value on Mac2Sell is really a useless exercise.
They are coming up with a single number which is simply a rough guideline and tends to be the lowest price one can expect or the best deal a buyer can make.
Last time this was discussed I checked a whole set of completed ebay auctions which give you the range of prices a specific MacBookPro was sold for. At that time everyone also complained that the Mac2Sell price calculated was too low, but there were actual sales on ebay at the Mac2Sell price and even slightly lower - but also sales hundreds of dollars more.
The actual selling price of a particular MacBookPro, or any item for that matter, depends on many factors and can vary quite a bit, even from day to day.
I think Mac2Sell would be better of to come up with a range of prices rather than just a single number.