Actually the people that buy the "old" iphone before iPhone Version 2 is released are very knowledgeable.
These people are probably business people or just have so much money that saving $100-200 to shop around for an unlock iPhone isn't worth their trouble. Believe me, I'm in the medical profession and if I take a day off to go to court for a traffic ticket, I'm losing $1500-2500 for the day lost of work. So while I wouldn't like to overpay for an iPhone, if I didn't know how to take take to unlock the iPhone and I didn't want to waste my time to research it, I would just pay the extra money just to get the device.
I sold my 8GB iPhone (it was my 3rd refurbished iPhone) for $325 on craigslist in 10 minutes. Better to get some money than none and apply it to the next iPhone whenever it comes out.