Over time I find it pretty accurate if you only look at the "sold" prices. There is an economic model for auction pricing, the essense is that the true value of an object is the amount 2 people are prepared to pay. This is because the winning bidder may have over paid and so the value is what they can sell for, which is the amount of the loosing bid since that bidder would pay that price to have it. So the value is taken to be the last loosing bid since this is a value that at least 2 people agree on. E-bay uses graded increments so its not always obvious what the loosing bid was but if you drop down a % or 2 from the sold price you won't be far off.