SELECT SUM(Stddev/AvP * cnt)/SUM(cnt) FROM (
SELECT Brand, Family, Series, MAX(Price) AS MaxP, AVG( Price ) AS AvP, MIN(Price) AS MinP, STDDEV(Price) AS Stddev, Count(ItemID) AS cnt
FROM training_data
WHERE LENGTH(Series) > 0 AND Series != "XP"
GROUP BY Brand, Family, Series, CPUType, CPUCores, CPUSpeed, RAM, HDD, LCD, OS, Drive, ItemCondition
HAVING (Count(ItemID) > 1)) AS Equivalences

Result:
0.088

Which means the following:
A difference in eBay laptop prices for exactly the same laptops is 8.8%

/>Example:
You sell Dell D600 for $200
You sell exactly the same Dell D600 for $230
/>Average is (200+230)/2 = $215.
/>Standard deviation is sqrt(sqr(200-215)+sqr(230-215)) = 21.21
Standard deviation in terms of average price is 21.21/215 = 0.098 = 9.8%
That's what this huge SQL thing on top measured, for all sets of the same laptops.