I find that quite odd. We've always found Everymac to be a decent basis to work off. Not always 100% accurate but a better figure than eBay guesswork
I will admit there was a time (a longgg time ago)...before I got more knowledgeable of used Macintosh prices...I used to rely on the pricing at Everymac.com. If you search Everymac.com for every model Mac Apple has released for the last 5-10 years...you may find some values that are accurate. But I think that this is more coincidence than anything else.
I'll give you another VERY inaccurate value Everymac.com is estimating. Take a dual Powermac G5 1.8ghz. Everymac.com says it's worth $450-$800 here in the United States.
Power Macintosh G5 1.8 DP (PCI-X) Specs (Original/Late 2003, M9393LL/A, PowerMac7,2, A1047, 1969) @ EveryMac.com
That is absolutely insane!! I know for a fact that 1.8ghz dual processor Powermac G5's sell on e-Bay everyday for around $250. Sometimes you can get them as low as $200...up to $300 if they have a a lot of ram & a great video card.
Let's not even talk about how inaccurate that $800 figure is. For $800 dollars...I can buy soooo much more computer than a 7-8 year-old Powermac G5. Heck for $999...I can buy a brand new MacBook...refurbished I can get one for $849.
As far as e-Bay...there is no "guesswork"! You simply "watch" 10-20 auctions for the same model computer (similar specs, ram HD, cpu, video, etc.). When all 10-20 auctions are over...you look at the values...and you have a pretty good idea of what a specific model Macintosh computer is worth.
- Nick
p.s. Here's another example. A 2.0ghz Core 2 Duo MacBook from 2006. Everymac.com lists it's value as $650 to $750:
http://www.everymac.com/systems/apple/macbook/stats/macbook-core-2-duo-2.0-white-13-specs.html
I "MIGHT" consider the $650 value as a possible "upper limit" for this computer (rather than the lower limit)...but only for a REALLY NICE well taken care of unit...with probably a ram & HD upgrade. I personally have purchased newer MacBooks (2007 or 2008 models) for $350-$450 from my local Craig's List.
But again...why would I pay $750 for a 4-5 year-old 2006 MacBook...when I can buy this MUCH newer refurbished MacBook (2.4ghz Core 2 Duo) from the Apple Store for $849 with a full 12-month warranty:
http://store.apple.com/us/product/FC516LL/A?mco=MTg0NzA2MTU