The 3G iPhone requires in-store activation. Therefore, unlocked 3G iPhones are going to be slightly rarer (in order to get one for the cheapest price, you need to buy the 3G iPhone for $199/$299, then wait 30 days, then cancel and pay termination fee [which taints your account]...then you'd need to donate one to the hackers so they can unlock it).
The current iPhone can be unlocked really easily...especially since you activate it at home.
And to be honest, a lot of eBay users aren't the brightest.
And don't forget...the 3G iPhone also raises the data plan to $30 and takes out the 200 SMS.
Some people just don't need 3G or GPS to pay an extra $10 and give up texting.